The
 history of the Catholic Church is full of all sorts of heresies that 
have assailed the truths of the faith.  From the earliest days of the 
Gnostics and Docetists all the way down to the Jansenists and Quietists 
of later centuries, it seems there has never been a shortage of 
heretical thought.
But in each age, God has brought 
forth great members of the faithful to combat each one.  Each one gave their life in 
service to Christ and His Church in their own way, either as martyrs, 
confessors, or simply as servants to others for the sake of the love of 
Jesus.
The following is a list of fifteen of the major 
heresies that the Church has faced, and the illustrious persons who stood
 against them. 
1.  Pelagianism and St. Augustine of Hippo
"There is an
 opinion that calls for sharp and vehement resistance - I mean the 
belief that the power of the human will can of itself, without the help 
of God, either achieve perfect righteousness or advance steadily towards
 it."1 
Pelagianism radically 
corrupted the Church's teachings on grace, sin, and the Fall.  Its 
namesake, the British monk Pelagius (who was startled by some of the words of St. Augustine in his 
Confessions), taught that the sin of Adam had no bearing on 
subsequent generations; essentially, man was inherently good and 
unaffected by the Fall.  In practice, this meant that a man could come 
to God by his own free will, no grace needed.  Many saints fought 
against this doctrine - St. David of Wales stands out among them 
especially - but it was St. Augustine of Hippo, arguably the greatest of 
the Latin Doctors and "the Church's mightiest champion against heresy"
2, who rose to fight against this inherently venomous strand of thought.
Against
 Pelagius, St. Augustine upheld the truth that God's grace is entirely 
necessary for any movement of ours towards God to occur at all.  As he 
himself puts it, "We for our part assert that the human will is so 
divinely aided towards the doing of righteousness that, besides being 
created with the free choice of his will, and besides the teaching which
 instructs him how he ought to live, he receives also the Holy Spirit, 
through which there arises in his heart a delight in and love of that 
supreme and unchangeable Good which is God; and this arises even now, 
while he still walks by faith and not by sight."
3
 2.  Gnosticism and St. Irenaeus of Lyons
"How
 can they say that the flesh goes to corruption and has no share in 
life, when it is nourished by the Lord's Body and Blood?"4
2.  Gnosticism and St. Irenaeus of Lyons
"How
 can they say that the flesh goes to corruption and has no share in 
life, when it is nourished by the Lord's Body and Blood?"4
Gnosticism
 was arguably the biggest heresy of the early Church, a Hydra-like 
species of varying sects and figureheads that espoused all manner of 
profane mysticism, asceticism, and produced many false gospels.  Among 
its central tenets was that Christ was merely a spiritual being, and not
 a flesh-and-blood man, that God the Father was actually a malevolent 
Demiurge, and that all matter was inherently evil.
The 
chief saint who fought Gnosticism, and dismantled all aspects of it was 
St. Irenaeus of Lyons.  St. Irenaeus' monumental work, 
Adversus Haereses,
 is a systematic account and refutation of every Gnostic sect presumably
 known by St. Irenaeus at the time.  He tenaciously held that Christ was God 
in the flesh, for if Christ was merely a phantasm, then He did not 
suffer and die at all.  His writing is essential for understanding the 
heresies that assaulted the Church in the first two centuries of its 
existence, as well as being an incredible account of apostolic tradition
 up to his time.
3.  Arianism and St. Athanasius
"And
 thus, taking a body like to ours, because all men were liable to the 
corruption of death he surrendered it to death instead of all, and 
offered it to the Father..."5
Aside
 from the various Gnostic sects that plagued the early Church, it is 
Arianism that is arguably the most famous of all Christian heresies.  It
 struck at the very root and core of Christian teaching, that Jesus was 
God Himself in the flesh, and relegated the person of Jesus Christ to 
that of a mere created thing.  It lives on today in varying forms, from 
well-known sects like the Jehovah's Witnesses all the way to the bizarre
 world of Apollo Quiloboy; moreover, it still lurks within the sentences
 of some modern theologians who ambiguously state that Jesus is "the 
Christ" but no more than an exalted man.
St. Athanasius
 of Alexandria was the walking cure for this heresy.  Stubborn and 
unshakeable, I think it not a stretch to say at times that this great 
man stood alone against wave after wave of Arian attacks on the truth of
 the Christian faith.  By emphasizing and stubbornly holding to the 
truth of Christ as both God and man, St. Athanasius (along with others 
such as St. Hilary of Poitiers) effectively ended the reign of the Arian
 heresy within the Church.  
4.  Nestorianism and St. Cyril of Alexandria
"Truth reveals herself plain to those who love her."6
St.
 Cyril of Alexandria was not known for his subtlety when it came to 
those who would attack the revealed truth of the Christian faith.  When 
Nestorius arose on the scene, Pope St. Celestine I sent St. Cyril to 
quell the heresies spread by this man.  Nestorius' error was essentially
 (and might I say,  ironically) two-fold: the Blessed Virgin Mary was 
not the Mother of God but merely the 
Christotokos (meaning 
"Christ-bearer")  and who also effectively claimed that Christ was 
really two persons accidentally united in one body (one divine, one 
human). 
Against this, St. Cyril defended the unity of 
Christ's person as both God and man with a ferocity that I have 
personally not witnessed in writing since St. Jerome defended the 
perpetual virginity of the Virgin Mary against Helvidius in 383 AD.  St.
 Cyril's brilliant defense of the person of Christ at the Council of 
Ephesus forever set up an impenetrable fortress against all those who 
would attack both the Incarnation and the Mother of God. 
5.  Monothelitism and St. Maximus the Confessor
"I have the faith of the Latins, but the language of the Greeks."7
Monothelitism
 declared that Christ had only one will (divine).  Much like 
Monophysitism which had declared that Christ had only one nature 
(divine), Monothelitism is viewed by some as a compromise aimed at 
bringing Monophysites back to the Church.  But by declaring that Christ 
had only a divine will, it amounted to little more than essentially 
stating that Jesus was not God in flesh but merely a human controlled by
 a divine will - Justin Holcomb of the Reformed website 
The Resurgence humorously describes it as "Jesus is controlled by Skynet"
8.
Against
 this heresy arose the valiant St. Maximus the Confessor, who is to this
 day one of the most revered theological minds of the Christian East.  
His defense of the orthodox doctrine that Christ had both a human will 
and divine will was met with fearsome resistance - he ended up having 
his tongue torn out and his right hand cut off for refusing to acquiesce
 to the Monothelite Emperor Constans II, before being exiled and dying 
soon after.  
6.  Albigensianism and St. Dominic Guzman
"...his
 heart was well-nigh broken by the ravages of the Albigensian heresy, 
and his life was henceforth devoted to the conversion of heretics and 
the defence of the faith."9
Gnosticism
 again reared its ugly head in the Middle Ages, this time in the form of
 what was known as Albigensianism.  With its dualist worldview and 
inherent dislike for the Church due to corruption within her own ranks 
among the clergy, Albigensianism began to attract an incredibly large 
following, divided into the "perfect" and "believers." Though often 
romanticized nowadays due to the revival of interest in Gnostic ideas 
and history within the New Age movement, from my point of view, it was 
anything but.  In fact, it was alarming in its view of all matter as 
evil - suicide by starvation was encouraged among its members, in order 
to free the soul from the body.  In fact, when a run-of-the-mill 
"believer" was given the spiritual baptism whilst seriously ill and/or 
dying, and happened to recover somehow, they were "as often as not 
smothered or starved to death (
endura) in order to assure [their] salvation,"
10 because only once could this ritual be performed.
Though
 the Cistercian order had been enlisted to combat this heresy, its 
success was minimal at best.  St. Dominic instead founded the Order of 
Preachers, because in all practicality "what was needed was a new policy
 with missioners travelling in poverty, but well-equipped intellectually
 to deal with the errors in a charitable but effective way."
11 
 The accounts surrounding his battles against the heresy of the Cathari 
(as the Albigensians were also known) are incredible - his staying up 
all night in discussion with an Albigensian innkeeper in order to save 
his soul, the Virgin Mary's arming him with the Holy Rosary, his singing
 hymns aloud along the roads where Cathari assassins lay in wait to 
murder him (much to their astonishment!), his only book that he carried 
being a copy of the Gospel of St. Matthew.  It is even said of the 
Dominicans that "Our Lady took them under her special protection, and 
whispered to St. Dominic as he preached."
12
Though
 the murder of a papal legate by the Albigensians sparked a massacre in 
the form of the Albigensian Crusade, "Dominic himself took no part in 
the violence of the crusaders."
13  In the end, due
 to his zeal for, love of, and devotion to Christ, "he revived the the 
courage of the Catholic troops, led them to victory against overwhelming
 numbers, and finally crushed the heresy."
14
 7.  Latin Averroism and St. Thomas Aquinas
"This then is what we have written to destroy the error mentioned, using the arguments and
teachings of the philosophers themselves, not the documents of faith. If anyone glorying in the
name of false science wishes to say anything in reply to what we have written, let him not speak
in corners nor to boys who cannot judge of such arduous matters, but reply to this in writing, if
he dares. He will find that not only I, who am the least of men, but many others zealous for the
truth, will resist his error and correct his ignorance."15
7.  Latin Averroism and St. Thomas Aquinas
"This then is what we have written to destroy the error mentioned, using the arguments and
teachings of the philosophers themselves, not the documents of faith. If anyone glorying in the
name of false science wishes to say anything in reply to what we have written, let him not speak
in corners nor to boys who cannot judge of such arduous matters, but reply to this in writing, if
he dares. He will find that not only I, who am the least of men, but many others zealous for the
truth, will resist his error and correct his ignorance."15
One
 does not exactly hear of the movement known as Latin Averroism too much
 these days.  But it was indeed a kind of heresy, if you will, a school 
of thought that attacked the truth of Christian dogma and belief at its 
core.  Influenced by the Islamic philosopher Averroes (Ibn Rushd, 
labelled by the Scholastics as "the Commentator" due to his extensive 
commentaries on Aristotle), the Averroist Scholastics taught a kind of 
double truth.  For the Averroist, something that was true in religion 
and theology could be at the same time false in philosophy and 
practicality.  Mixed in with this paradoxical notion of "true and not 
true at the same time", the Averroists also held that the world had 
always existed, and that there was only one collective soul in humanity.
Against
 this school of thought, St. Thomas Aquinas rose like a mighty fortress 
to protect Holy Mother Church.  Instead of outright dismissing the 
thought of Aristotle like some (due to its being associated with this 
new movement in thought, as well as some of Aristotle's ideas 
themselves), St. Thomas Aquinas answered the Averroists by using 
Aristotle himself.  With precision and common sense, the Angelic Doctor 
pointed out the corruptions in the translations of Aristotle used by the
 Arab philosophers, corrected abuses of Aristotle's thought, and 
harmonized faith and reason rather than separating them into two spheres
 of truth.  All in a day's work for one of the greatest minds the Church
 has ever known.
8.  Calvinism and St. Francis de Sales
"In
 fact I thought that as you will receive no other law for your belief 
than that interpretation of the Scripture which seems to you the best, 
you would hear also the interpretation that I should bring, viz., that 
given by the Apostolic Roman Church, which hitherto you have not had 
except perverted and quite disfigured and adulterated by the enemy, who 
well knew that had you seen it in its purity, never would you have 
abandoned it."16 
In 
the inital aftermath of the Reformation, the varying schools of 
Protestantism had begun to take root.  But none had shown themselves to 
be as staunch in resisting the Catholic faith as the followers of John 
Calvin.  Though he makes extensive use of the thought of St. Augustine, 
he does so with hardly any reference to the rest of the Fathers (even a 
cursory glance at an index in a copy of his magnum opus, the
 
Institutes of the Christian Religion, shows this), ignoring "all that Catholic foundation on which the Doctor of Grace built."
17 
Enter
 St. Francis de Sales.  Only 27 years old at the time, he was sent into 
one of the most anti-Catholic regions of all, the Chablais, wherein 
Calvinism had especially fortified itself.   To do so was to invite 
being despised, rejected, misunderstood, threatened, and turned away.  
In many respects, St. Francis' missions to the Calvinists call to mind 
the words of St. Paul himself - 
"I have been on frequent journeys, in dangers from rivers, dangers from robbers, dangers from my
 countrymen, dangers from the Gentiles, dangers in the city, dangers in 
the wilderness, dangers on the sea, dangers among false brethren; I
 have been in labor and hardship, through many sleepless nights, in 
hunger and thirst, often without food, in cold and exposure.  Apart from such external things, there is the daily pressure on me of concern for all the churches.  Who is weak without my being weak? Who is led into sin without my intense concern?" (2 Cor. 11:26-29)
With the Calvinist population staunchly refusing to 
listen to his words, St. Francis began to write and distribute pamphlets
 on the truth of the Catholic faith.  These writings were compiled later
 on into one work, probably the greatest apologetic work against 
Protestant objections ever penned - Les Controverses.  Known as "the gentleman saint", St. Francis' untiring love for souls (especially seen in his other great work, Introduction to the Devout Life),
 his knowledge of the faith and history, and his incredible ability to 
adapt and endure all manner of obstacles and hardship sent against him 
make him arguably the greatest of the Doctors who went forth against the
 errors of Calvinism.
9.  Monophysitism and Pope St. Leo the Great
"Keep your hearts free, my beloved, from poisonous lies inspired by the devil."18
Monophysitism
 was essentially the opposite of the Nestorian heresy mentioned above; 
where Nestorius emphasized that in Christ "there was both a human 
hypostasis or person and a divine"
19, the 
Monophysite heresy declared that Christ had only one nature, that His 
humanity was absorbed into His divinity.  While the heresy of Nestorius 
was largely vanquished twenty years earlier by St. Cyril of Alexandria 
at the Council of Ephesus, it was Pope St. Leo the Great who arose to do
 battle with the heresy of Eutyches and the Monophysites.
Against
 Monophysitism, he taught the truth of the two natures of Christ (human 
and divine), saying of Christ that "we could not overcome the author of 
sin and death, unless He had taken our nature and made it his own..."
20. 
 "After three years of unceasing toil, Leo brought about its solemn 
condemnation by the Council of Chalcedon, the fathers all signing his 
tome, and exclaiming, 'Peter hath spoken by Leo.'"
21
10.  Iconoclasm and St. John of Damascus
"Conquest is not my object.  I raise a hand that is fighting for the truth - a willing hand under the divine guidance."22
Iconoclasm, the rejection of the use of religious imagery in worship 
(icons, statuary, and even extending to the use of candles, incense, 
etc.) had a complicated history.  In the early centuries, it was to be 
found amongst the heretical Paulician and Nestorian camps, but it was 
also espoused by some within the Church (including, very early on, St. 
Epiphanius of Salamis who "fell into some mistakes on certain occasions,
 which proceeded from zeal and simplicity."
23).  
Moreover, the heresy of Iconoclasm found much of its influence and fuel 
in the rise of Islam, which was fiercely opposed to the use of imagery 
in worship.
The chief heretic in this struggle was 
Emperor Leo II the Isaurian, who issued an edict forbidding the use of 
imagery in religious worship.  St. John Damascene, considered the last 
of the Greek Fathers and the first of the Scholastics, immediately set 
to work defending the use of imagery by Christians since the earliest 
centuries of the Church.  St. John was arrested by the Emperor, and 
(much like St. Maximus the Confessor) had his right hand severed as a 
punishment for his resistance to the heresy by way of his writings.  
Iconoclasm was eventually condemned by the Second Council of Nicaea in 
787, but was resurrected again in the Protestant Reformation.
11.  Jansenism and St. Alphonsus de Liguori
"He who does not acquire the love of God will scarcely persevere in the 
grace of God, for it is very difficult to renounce sin merely through 
fear of chastisement."24
The
 errors of Calvinism were not only to be found within the Protestant 
realm, but within the Church too did they take root as well.  This 
Catholic/Calvinist hybrid was founded by the theologian Cornelius 
Jansen, who, like Calvin, took the writings of St. Augustine and ran 
with them to the most extreme conclusions.  A species of ridiculous 
moral rigorism and religious fear spread its shadows over the Church.  
It discouraged frequent Holy Communion, espoused a form of moral 
perfectionism as being a 
requirement to even receive the 
Eucharist at all.  So successful was its influence that it even found 
adherents in such brilliant Catholic minds as Blaise Pascal.
Many
 great men and women stood firm against the pessimistic theology and 
destructive results of Jansenist doctrine, but it was St. Alphonsus de 
Liguori's writings and thought which effectively sounded the death-knell
 of this particular form of heresy.  Against the rigorism and fear 
espoused by Jansenism, St. Alphonsus encouraged frequent Holy Communion 
as a remedy for sin as long as one was not in a state of 
mortal 
sin, and developed a finely-tuned moral theology that became the 
standard textbook of all Catholic moral theology since.  He is to this 
day not only revered as a Doctor of the Church and founder of the 
Redemptorist order, but as the most excellent of teachers on the subject
 of Catholic morality.
12.  Brethren of the Free Spirit and Bl. John of Ruysbroeck
"This is that Wayless Being which all fervent interior spirits have 
chosen above all things, that dark stillness in which all lovers lose 
their way. If we could prepare ourselves through virtue in the ways I have shown, we
 would at once strip ourselves of our bodies and flow into the wild 
waves of the Sea, from which no creature could ever draw us back."25
The
 heresy of the Brethren of the Free Spirit is not one that much heard of
 these days, but its influence is more widespread than is commonly 
known.  Finding its beginnings in the Beguine and Beghard movement in 
the 13th and 14th centuries, this heretical movement found major 
inspiration in the sermons and writings of Meister Eckhart (though he 
himself denied any involvement with the movement).  Emphasizing a form 
of indifference to salvation (a kind of proto-quietism), union with God 
in this life, and attacking the sacraments of the Church, this 
mystically-charged heresy began to spread itself all about central 
Europe.
Though some of the followers of Meister 
Eckhart himself (especially Bl. Henry Suso) either denied involvement 
with the Free Spirit movement and/or attempted to correct its teachings 
and combatted its influence with that of orthodox mysticism within the 
bounds of the Church, it was the greatest of the Flemish mystics, Bl. 
John of Ruysbroeck, that led the charge against this particular brand of
 mystical heresy.
The life of Bl. John is a 
fascinating one to peruse - spending much of his time in prayer and 
contemplation in the Sonian Forest near Groenendaal, his concern for the
 welfare of souls being led astray by the quietistic Free Spirit 
movement was such that he began to engage in open theological combat 
with them.  His writings are some of the best ever penned on the Holy 
Trinity, as well as on the mystical life.  Instead of writing 
linguistically remote treatises that could never be accessed by the 
average person at the time, Bl. John wrote many pamphlets in the 
vernacular that defended the faith against heretical attacks by such 
Free Spirit figureheads as Bloemardinne.  By emphasizing the deepest 
aspects of mysticism within Church orthodoxy, he effectively brought 
about the end of this movement, though not without being persecuted 
intensely by adherents of this heresy.  
13.  Modernism and Pope St. Pius X
"That We make no delay in this matter is rendered necessary especially by the
fact that the partisans of error are to be sought not only among the Church's
open enemies; they lie hid, a thing to be deeply deplored and feared, in her
very bosom and heart, and are the more mischievous, the less conspicuously they
appear. We allude, Venerable Brethren, to many who belong to the Catholic laity,
nay, and this is far more lamentable, to the ranks of the priesthood itself,
who, feigning a love for the Church, lacking the firm protection of philosophy
and theology, nay more, thoroughly imbued with the poisonous doctrines taught by
the enemies of the Church, and lost to all sense of modesty, vaunt themselves as
reformers of the Church; and, forming more boldly into line of attack, assail
all that is most sacred in the work of Christ, not sparing even the person of
the Divine Redeemer, whom, with sacrilegious daring, they reduce to a simple,
mere man."26
Modernism is quite possibly the most 
controversial heresy mentioned on this list, because we are indeed, 
right up to this very moment, still in the throes of it.  As for my own 
view, it seems to me to be the most ambiguous and chameleon-like of all 
heresies, and it can often be hard to pinpoint exactly where it is 
entrenched or where it has already passed through and damaged the 
faith.
Modernism seems to have had its beginnings, 
somewhat officially, in the 19th century.  Figures such as Maurice 
Blondel, George Tyrrell, Alfred Loisy, Friedrich von Hugel and many 
others are considered major figures within the movement within the 
Catholic Church; in Protestantism, I would argue that much of it was to 
be found initially in the thought of Friedrich Schleiermacher.
The words of the modernist thinkers themselves is especially startling -
 Alfred Loisy wrote that "Christ has even less importance in my religion
 than he does in that of 
the liberal Protestants: for I attach little importance to the 
revelation of God the Father for which they honor Jesus. If I am 
anything in religion, it is more pantheist-positivist-humanitarian than 
Christian."
27
Its effects are 
highly destructive - central to it is the idea that the truths of the 
Christian religion must be subjected to Enlightenment-style rationalism,
 relativism and secularism.  The truths of the ancient faith are viewed 
as outmoded, and consequently subjected to rigorous demythologization.  
Additionally, the notion of the evolution of dogma effectivelly brought 
to bear a devastating assault on the truths of the Christian religion.
The
 effects of a modernistic viewpoint are seen to this day in much 
theological thought, both Protestant and Catholic, in the writings of 
many major thinkers such as Hans Kung, Edward Schillebeeckx, Rudolf Bultmann, Karl Rahner, and a whole 
host of others.  The status of whether many theologians and writers are 
actually modernistic is a hotly-debated topic.
On
 the Protestant end of it, it was resisted mightily by the Reformed 
theologian Karl Barth, especially in his clarion call against liberal 
theology entitled 
The Epistle to the Romans.  Though beforehand, the 
Syllabus of Errors of 1864 and the encyclical of Pope Leo XIII entitled 
Providentissimus Deus had begun to defend the Church against Modernism,
 it was the great Pope St. Pius X who arose as the greatest defender of 
the Church by warning of modernism's threat to the faith.
Calling it the "synthesis of all heresies"
28, Pope St. Pius X released 
Lamentaboli Sane (Syllabus Condemning the Errors of the Modernist) and his monumental encyclical 
Pascendi Dominici Gregis
 against the modernist school of thought.  Reading the work is a 
frightening wake-up call to the insidious nature of the heresy itself - 
unlike the dangerous yet frankly clumsy assaults of earlier heresies 
upon the faith such as Arianism and Montanism, Modernism was said to 
have infected the Church from the inside.  One is reminded of a deadly 
illness more than an attack.
Pope St. Pius X also wrote the famed 
Oath Against Modernism which was required to be sworn to by clergy and others in the Church,
 and sought to warn the faithful before it was too late.  Much work was 
done to extinguish modernist trends of thought within the Church thanks 
to this most venerable and saintly Pope, and to this day, he remains the
 most important saint to have ever fought against the poisonous 
infections of the movement.  
14.  Origenism and St. Methodius of Olympus
"Shun not, man, a spiritual hymn, nor 
be ill-disposed to listen to it. Death belongs not to it; a story of 
salvation is our song."29
Without
 a doubt, the Alexandrian theologian Origen was the greatest 
mind of the early Church.  Many of the great saints of the early Church 
were enthralled by his brilliance and his devotion - I would make 
mention of St. Basil the Great, St. Gregory Nazianzen, and St. John 
Cassian especially.  Even St. Jerome, who became a bitter opponent of 
Origen's thought later on, still held him to be one of the most 
admirable and brilliant minds the Church had yet known.  St. Francis de 
Sales and St. Elizabeth Schonau, writing many centuries later, also 
spoke of his great services to the Church.
Nevertheless, 
some of the thought of Origen was exceedingly problematic.  Being one of
 the first theologians proper of the early Church, he was prone to 
stumble when going too far deep into the truths of the faith.  His 
tendency to over-allegorize, his teachings on the pre-existence of 
souls, amongst other things, ended up getting him into trouble later on.
But,
 in all fairness to Origen, there is a huge difference between the man 
and what later came to be known as "Origenism".  Origenism took latent 
elements in the experimental and speculative thought of Origen and often
 ran with it, much in the same manner, I would argue, as such men as 
John Calvin and Cornelius Jansen had done with the thought of St. 
Augustine.
Several saints began to criticize 
Origenism as such, notably St. Jerome and St. Epiphanius of Salamis.  
But the first to systematically attack the errors in Origen's thought 
was one St. Methodius of Olympus.  Himself well-trained in Platonist 
philosophy as well as the theology of the Church, St. Methodius 
vigorously critiqued the major errors in the thought of the great 
Alexandrian, including the eternity of the world and certain teachings 
of his on the resurrection.  Though a devoted opponent of the thought of
 Origen, it is interesting to note that he still recognized his service 
to the Church.
The errors of Origenism were finally condemned at the Second Council of Constantinople in 553 AD, though 
The New Catholic Encyclopedia
 promulgated under the pontificate of Pope Pius XI says that "it is not 
proved that he incurred the anathema of the Church at the Fifth General 
Council."
15.  Religious Indifferentism and Pope Pius XI
"For union of Christians can only be promoted by promoting the return to
 the one true Church of Christ of those who are separated from it, for 
in the past they have unhappily left it. To the one true Church of 
Christ, we say, which is visible to all, and which is to remain, 
according to the will of its Author, exactly the same as He instituted 
it."31
Religious 
indifferentism is, in essence, a kind of sub-species of modernism.  It 
undermines the truth of the Catholic Church as the one true Church 
founded by Christ, and essentially states that it is a matter of 
indifference which church one belongs to.  In many ways, it amounts to what might be termed "pan-Christianity".   
Against this notion, Pope Pius XI wrote the encyclical entitled 
Mortalium Animos,
 which again underlined that the Catholic Church was the Ark of 
Salvation, and attacked the idea of a kind of watered-down pan-Christian
 collective of churches.  All that it amounts to, in essence, is a unity
 based upon false ecumenism, a kind of "whatever" pseudo-Christianity.  
This religious indifferentism essentially espouses the notion that 
"Controversies... and longstanding
differences of opinion which keep asunder till the present day the
members of the Christian family, must be entirely put aside, and
from the remaining doctrines a common form of faith drawn up and
proposed for belief, and in the profession of which all may not only
know but feel that they are brothers."
32
Though many had condemned religious indifferentism beforehand (Pope Leo 
XIII, Pope Gregory XVI, Pope Benedict XV, as well as the 1864 
Syllabus of Errors),
 it was Pope Pius XI who decisively defended the Church against it, 
quoting the early Church Father Lactantius: "The Catholic Church is 
alone in keeping the true worship. This is the 
fount of truth, this the house of Faith, this the temple of God: if any 
man enter not here, or if any man go forth from it, he is a stranger to 
the hope of life and salvation. Let none delude himself with obstinate 
wrangling. For life and salvation are here concerned, which will be lost
 and entirely destroyed, unless their interests are carefully and 
assiduously kept in mind."
33 
1 - 
The Spirit and the Letter, IV
2 - 
Butler's Lives of the Saints, "St. Augustine of Hippo", 1894 edition 
3 - 
The Spirit and the Letter, V
4 - 
Against the Heresies, IV:18:5 
5 - 
On the Incarnation, VIII
6 - 
Second Letter to Succensus, I 
7 - From 
here. 
8 - From 
here.
9 - 
Butler's Lives of the Saints, "St. Dominic", 1894 edition 
10 - Rev. John Laux, 
Church History, IV:1 
11 - David Farmer, 
Oxford Dictionary of Saints, "Dominic", pg. 146
12 - 
Butler's Lives of the Saints, "St. Dominic", 1894 edition
13 - David Farmer, 
Oxford Dictionary of Saints, "Dominic", pg. 146
14 - ibid.
15 - 
De Unitate Intellectus Contra Averroistas, 124
16 - 
The Catholic Controversy, "Author's General Introduction" 
17 - William Barry, 
The Catholic Encyclopedia, "Calvinism"
18 - "Sermon 28", VI 
19 - 
The New Catholic Dictionary, "Monophysites and Monophysitism"
20 - 
Ep. xxviii, II 
21 - 
Butler's Lives of the Saints, "St. Leo the Great", 1894 edition
22 - 
On Holy Images, I 
23 - 
Butler's Lives of the Saints, "St. Epiphanius of Salamis", 1894 edition
24 - From 
here.
25 - 
The Spiritual Espousals, found 
here.
26 - 
Pascendi Dominici Gregis, 2
27 - Memoires II, pg. 397
28 - 
Pascendi Dominici Gregis, 39
29 -
 Concerning Free Will
30 - 
The New Catholic Encyclopedia, "Origenism"
31 - 
Mortalium Animos, 10
32 - ibid., 7
33 - Lactantius, 
Divine Institutes, IV:30:11-12, 
cf. Pope Pius XI, 
Mortalium Animos, 11