The following quotes are from The Commonitory by Saint Vincent of Lerins. Given recent discussions that have taken place, I thought they would be good to post.
Chapter 3
"What then will a Catholic Christian do, if a small portion of the Church have cut itself off from the communion of the universal faith? What, surely, but prefer the soundness of the whole body to the unsoundness of a pestilent and corrupt member? What, if some novel contagion seek to infect not merely an insignificant portion of the Church, but the whole? Then it will be his care to cleave to antiquity, which at this day cannot possibly be seduced by any fraud of novelty."
Chapter 4
"So also when the Arian poison had infected not an insignificant portion of the Church but almost the whole world, so that a sort of blindness had fallen upon almost all the bishops of the Latin tongue, circumvented partly by force partly by fraud and was preventing them from seeing what was most expedient to be done in the midst of so much confusion, then whoever was a true lover and worshipper of Christ, preferring the ancient belief to the novel misbelief, escaped the pestilent infection."
Chapter 6
"For it has always been the case in the Church, that the more a man is under the influence of religion, so much the more prompt is he to oppose innovations."
Chapter 9
"To preach any doctrine therefore to CatholicChristians other than what they have received never was lawful, never is lawful, never will be lawful: and to anathematize those who preach anything other than what has once been received, always was a duty, always is a duty, always will be a duty."
Chapters 10-11
"The reason is clearer than day why Divine Providence sometimes permits certain doctors of the Churches to preach new doctrines--'That the Lord your God may try you,' he says. And assuredly it is a great trial when one whom thou believest to be a prophet, a disciple of prophets, a doctor and defender of the truth, whom thou hast folded to thy breast with the utmost veneration and love, when such a one of a sudden secretly and furtively brings in noxious errors, which thou canst neither quickly detect, being held by the prestige of former authority, nor lightly think it right to condemn, being prevented by affection for thine old master...
...For to take first a very recent and very plain case: what son of trial, think we, was that which the Church had experience of the other day, when that unhappy Nestorius, all at once metamorphosed from a sheep into a wolf, began to make havoc of the flock of Christ, while as yet a large proportion of those whom he was devouring believed him to be a sheep, and consequently were the more exposed to his attacks? For who would readily suppose him to be in error, who was known to have been elected by the high choice of the Emperor, and to be held in the greatest esteem by the priesthood? who would readily suppose him to be in error, who, greatly beloved by the holy brethren, and in high favor with the populace, expounded the Scriptures in public daily, and confuted the pestilent errors both of Jews and Heathens? Who could choose but believe that his teaching was Orthodox, his preaching Orthodox, his belief Orthodox, who, that he might open the way to one heresy of his own, was zealously inveighing against the blasphemies of all heresies? But this was the very thing which Moses says: 'The Lord your God doth try you that He may know whether you love Him or not.'...
...let us pass on to speak of those who, being persons of great attainments and of much industry, proved no small trial to Catholics. Such, for instance, was Photinus, in Pannonia, who, in the memory of our fathers, is said to have been a trial to the Church of Sirmium, where, when he had been raised to the priesthood with universal approbation, and had discharged the office for some time as a Catholic, all of a sudden, like that evil prophet or dreamer of dreams whom Moses refers to, he began to persuade the people whom God had intrusted, to his charge, to follow "strange gods," that is, strange errors, which before they knew not. But there was nothing unusual in this: the mischief of the matter was, that for the perpetration of so great wickedness he availed himself of no ordinary helps. For he was of great natural ability and of powerful eloquence, and had a wealth of learning, disputing and writing copiously and forcibly in both languages, as his books which remain. composed partly in Greek, partly in Latin, testify. But happily the sheep of Christ committed to him, vigilant and wary for the Catholic faith, quickly turned their eyes to the premonitory words of Moses, and, though admiring the eloquence of their prophet and pastor, were not blind to the trial. For from thenceforward they began to flee from him as a wolf, whom formerly they had followed as the ram of the flock."
Chapter 20
"...he is the true and genuine Catholic who loves the truth of God, who loves the Church, who loves the Body of Christ, who esteems divine religion and the Catholic Faith above every thing, above the authority, above the regard, above the genius, above the eloquence, above the philosophy, of every man whatsoever; who sets light by all of these, and continuing steadfast and established in the faith, resolves that he will believe that, and that only, which he is sure the Catholic Church has held universally and from ancient time; but that whatsoever new and unheard-of doctrine he shall find to have been furtively introduced by some one or another, besides that of all, or contrary to that of all the saints, this, he will understand, does not pertain to religion, but is permitted as a trial..."
Chapter 23
"For if once this license of impious fraud be admitted, I dread to say in how great danger religion will be of being utterly destroyed and annihilated. For if any one part of Catholic truth be given up, another, and another, and another will thenceforward be given up as a matter of course, and the several individual portions having been rejected, what will follow in the end but the rejection of the whole? On the other hand, if what is new begins to be mingled with what is old, foreign with domestic, profane with sacred, the custom will of necessity creep on universally, till at last the Church will have nothing left untampered with, nothing unadulterated, nothing sound, nothing pure; but where formerly there was a sanctuary of chaste and undefiled truth, thenceforward there will be a brothel of impious and base errors."
Chapter 24
"...it is an established law, in the case of almost all heresies, that they evermore delight in profane novelties, scorn the decisions of antiquity, and, through oppositions of science falsely so called, make shipwreck of the faith. On the other hand, it is the sure characteristic of Catholics to keep that which has been committed to their trust by the holy Fathers, to condemn profane novelties, and, in the apostle's words, once and again repeated, to anathematize every one who preaches any other doctrine than that which has been received."
Chapter 25
"It was for this reason that the Saviour cried, 'Beware of false prophets who come to you in sheep's clothing, but inwardly they are ravening wolves.' What is meant by 'sheep's closing'? What but the words which prophets and apostles with the guilelessness of sheep wove beforehand as fleeces, for that immaculate Lamb which taketh away the sin of the world? What are the 'ravening wolves'? What but the savage and rabid glosses of heretics, who continually infest the Church's folds, and tear in pieces the flock of Christ wherever they are able? But that they may with more successful guile steal upon the unsuspecting sheep, retaining the ferocity of the wolf, they put off his appearance, and wrap themselves, so to say, in the language of the Divine Law, as in a fleece, so that one, having felt the softness of wool, may have no dread of the wolf's fangs. But what saith the Saviour? 'By their fruits ye shall know them;' that is, when they have begun not only to quote those divine words, but also to expound them, not as yet only to make a boast of them as on their side, but also to interpret them, then will that bitterness, that acerbity, that rage, be understood; then will the ill-savour of that novel poison be perceived, then will those profane novelties be disclosed, then may you see first the hedge broken through, then the landmarks of the Fathers removed, then the Catholic faith assailed, then the doctrine of the Church torn in pieces."