Nothing can substitute for reading the entire text of the Commintory of St. Vincent of Lerins on heresy. It is available on several internet sites, e.g. at newadvent.org:
This is Part III, the last of the summary of Vincent’s work on heresies. In the excerpts below, a pithy comment in brackets precedes some paragraphs of Vincent’s own words.
“The Notes of a true Catholic.
[A single man does not and cannot determine the true faith]
“[48.] This being the case, 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 . . .
“Exposition of St. Paul’s Words.— 1 Tim. vi. 20.
[Wicked novelties of heresies will be decapitated with a spiritual sword]
“[51.] . . . I cannot sufficiently wonder at the madness of certain men, at the impiety of their blinded understanding, at their lust of error, such that, not content with the rule of faith delivered once for all, and received from the times of old, they are every day seeking one novelty after another, and are constantly longing to add, change, take away, in religion, . . . as with a spiritual sword, all the wicked novelties of all heresies often have been, and will always have to be, decapitated,
[Heretics teach furtively, and in secret; and tell us “silly wretches” they, despite centuries of error, now have the true faith]
“[52.] After words such as these, is there any one of so hardened a front, such anvil-like impudence, such adamantine pertinacity, as not to succumb to so huge a mass, not to be crushed by so ponderous a weight, not to be shaken in pieces by such heavy blows, not to be annihilated by such dreadful thunderbolts of divine eloquence? . . .Falsely called indeed, as applied to the doctrines of heretics, where ignorance is disguised under the name of knowledge, fog of sunshine, darkness of light. Which some professing have erred concerning the faith. Professing what? What but some (I know not what) new and unheard-of doctrine. For you may hear some of these same doctors say, Come, O silly wretches, who go by the name of Catholics, come and learn the true faith, which no one but ourselves is acquainted with, which same has lain hid these many ages, but has recently been revealed and made manifest. But learn it by stealth and in secret, for you will be delighted with it. Moreover, when you have learned it, teach it furtively, that the world may not hear, that the Church may not know. For there are but few to whom it is granted to receive the secret of so great a mystery. . .
“A more particular Exposition of 1 Tim. vi. 20.
[Heretics are thieves and adversaries]
“[53.] But it is worth while to expound the whole of that passage of the apostle more fully, O Timothy, keep the deposit, avoiding profane novelties of words. . . .
What is Keep the deposit? Keep it, because of thieves, because of adversaries, lest, while men sleep, they sow tares over that good wheat which the Son of Man had sown in his field. Keep the deposit.. . . . Keep the deposit. Preserve the talent of Catholic Faith inviolate, unadulterate. That which has been entrusted to you, let it continue in your possession, let it be handed on by you. You have received gold; give gold in turn. Do not substitute one thing for another. Do not for gold impudently substitute lead or brass. Give real gold, not counterfeit.
“On Development in Religious Knowledge.
[Adulteration of the faith is not real progress]
“[54.] But some one will say, perhaps, Shall there, then, be no progress in Christ’s Church? Certainly; all possible progress. For what being is there, so envious of men, so full of hatred to God, who would seek to forbid it? Yet on condition that it be real progress, not alteration of the faith. . . .
[The frenzy of the ungodly, the impious fraud. If one doctrine domino is allowed to fall, is contradicted, or is denied, all the doctrines fall; as does the Church itself]
“[58.] 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. May God’s mercy avert this wickedness from the minds of his servants; be it rather the frenzy of the ungodly.
“Continuation of the Exposition of 1 Tim. vi. 20.
[Shun him who proclaims heresy as you would a poisonous snake]
“[60.] But let us return to the apostle. O Timothy, he says, Guard the deposit, shunning profane novelties of words. Shun them as you would a viper, as you would a scorpion, as you would a basilisk, lest they smite you not only with their touch, but even with their eyes and breath. . . . Receive him not into your house, neither bid him Godspeed, for he that bids him Godspeed communicates with him in his evil deeds. 2 John 10
[Heretics do not speak clearly, and they cite Holy Scripture for their evil purposes]
“ [65.] But the more secretly they conceal themselves under shelter of the Divine Law, so much the more are they to be feared and guarded against. For they know that the evil stench of their doctrine will hardly find acceptance with any one if it be exhaled pure and simple. They sprinkle it over, therefore, with the perfume of heavenly language, in order that one who would be ready to despise human error, may hesitate to condemn divine words. . . .So too do these act, who disguise poisonous herbs and noxious juices under the names of medicines, so that no one almost, when he reads the label, suspects the poison.
[Like Satan, their father, heretics may seem to be of the light]
“ [67.] . . .And no wonder, he says, for Satan himself transforms himself into an angel of light. It is no marvel then if his servants are transformed as the servants of righteousness. Therefore, . . . there is no doubt that they are following the cunning devices of their father, which assuredly he would never have devised, but that he knew that where he could fraudulently and by stealth introduce error, there is no easier way of effecting his impious purpose than by pretending the authority of Holy Scripture.
[Heresy can be the newly-devised error of one man, who could put our eternal salvation in jeopardy]
“[72.] . … But whatsoever a teacher holds, other than all, or contrary to all, be he holy and learned, be he a bishop, be he a Confessor, be he a martyr, let that be regarded as a private fancy of his own, and be separated from the authority of common, public, general persuasion, lest, after the sacrilegious custom of heretics and schismatics, rejecting the ancient truth of the universal Creed, we follow, at the utmost peril of our eternal salvation, the newly devised error of one man.
[Be wary of an individual man who arrogantly proclaims that he alone has the truth]
“[74.] And lest any one, disregarding every one else, should arrogantly claim to be listened to himself alone, himself alone to be believed, the Apostle goes on to say, Did the word of God proceed from you, or did it come to you only? And, lest this should be thought lightly spoken, he continues, If any man seem to be a prophet or a spiritual person, let him acknowledge that the things which I write unto you are the Lord’s commands.”