With great zeal and closest attention, therefore, I frequently inquired of many men, eminent for their holiness and doctrine, how I might, in a concise and, so to speak, general and ordinary way, distinguish the truth of the Catholic faith from the falsehood of heretical depravity. I received almost always the same answer from all of them, that if I or anyone else wanted to expose the frauds and escape the snares of the heretics
who rise up, and to remain intact and sound in a sound faith, it would be necessary, with the help of the Lord, to fortify that faith in a twofold manner: first, of course, by the authority of the divine law; and then, by the Tradition of the Catholic Church.
Here, perhaps, someone may ask:
“If the canon of the Scriptures be perfect, and in itself more than suffices for everything, why is it necessary that the authority of
ecclesiastical interpretation be joined to it?”‘ Because, quite plainly, Sacred Scripture, by reason of its own depth, is not accepted by everyone as having one and the same meaning. The same passage is interpreted in one way by some, in another by others, so that it can almost appear as if there are as many opinions as there are men. Novatian explains a passage in one way, Sabellius in another, Donatus in another; Arius, Eunomius, Macedonius in another; Photinus, Apollinaris, Priscillian in another; Jovinian, Pelagius, Caelestius in another; and afterwards in still another, Nestorius
And thus, because of so many distortions of such various errors, it is highly necessary that the line of prophetic and apostolic interpretation be directed in accord with the norm of the ecclesiastical and Catholic meaning. [3] In the Catholic Church herself every care must be taken that we may hold fast to that which has been believed everywhere, always, and by all (1). For this is, then, truly and properly Catholic. That is what the force and meaning of the name itself declares, a name that embraces all almost universall. This general rule will be correctly applied if we pursue universality, antiquity, and agreement. And we follow universality in this way, if we contess this one faith to be true, which is confessed by the whole Church throughout the whole world; antiquity, however, if we in no way depart from those interpretations which, it is clear, our holy predecessors and fathers solemnized; and likewise agreement, if, in this very antiquity, we adopt the definitions and theses of all or certainly of almost all priests and teachers.
-Commonitoria 2:1

I have often then inquired earnestly and attentively of very many men eminent for sanctity and learning, how and by what sure and so to speak universal rule I may be able to distinguish the truth of Catholic faith from the falsehood of heretical pravity; and I have always, and in almost every instance, received an answer to this effect: That whether I or any one else should wish to detect the frauds and avoid the snares of heretics as they rise, and to continue sound and complete in the Catholic faith, we must, the Lord helping, fortify our own belief in two ways; first, by the authority of the Divine Law, and then, by the Tradition of the Catholic Church.
But here some one perhaps will ask, Since the canon of Scripture is complete, and sufficient of itself for everything, and more than sufficient, what need is there to join with it the authority of the Church’s interpretation? For this reason — because, owing to the depth of Holy Scripture, all do not accept it in one and the same sense, but one understands its words in one way, another in another; so that it seems to be capable of as many interpretations as there are interpreters. For Novatian expounds it one way, Sabellius another, Donatus another, Arius, Eunomius, Macedonius, another, Photinus, Apollinaris, Priscillian, another, Iovinian, Pelagius, Celestius, another, lastly, Nestorius another. Therefore, it is very necessary, on account of so great intricacies of such various error, that the rule for the right understanding of the prophets and apostles should be framed in accordance with the standard of Ecclesiastical and Catholic interpretation.
[6.] Moreover, in the Catholic Church itself, all possible care must be taken, that we hold that faith which has been believed everywhere, always, by all. For that is truly and in the strictest sense Catholic, which, as the name itself and the reason of the thing declare, comprehends all universally. This rule we shall observe if we follow universality, antiquity, consent. We shall follow universality if we confess that one faith to be true, which the whole Church throughout the world confesses; antiquity, if we in no wise depart from those interpretations which it is manifest were notoriously held by our holy ancestors and fathers; consent, in like manner, if in antiquity itself we adhere to the consentient definitions and determinations of all, or at the least of almost all priests and doctors.
-Commonitoria 2:4-6

But in this divine virtue, as we may call it, exhibited by these Confessors, we must note especially that the defense which they then undertook in appealing to the Ancient Church, was the defense, not of a part, but of the whole body. For it was not right that men of such eminence should uphold with so huge an effort the vague and conflicting notions of one or two men, or should exert themselves in the defense of some ill-advised combination of some petty province; but adhering to the decrees and definitions of the universal priesthood of Holy Church, the heirs of Apostolic and Catholic truth, they chose rather to deliver up themselves than to betray the faith of universality and antiquity.
-Commonitorium 5:14

“Nestorius, whose disease is of an opposite kind, while pretending that he holds two distinct substances in Christ, brings in of a sudden two persons, and with unheard-of wickedness would have two sons of God, two Christs,—one, God, the other, man; one, begotten of his Father, the other, born of his mother. For which reason he maintains that Saint Mary ought to be called, not the Mother of God, but the Mother of Christ” (The Notebooks 12[35] [A.D. 434]).

To preach any doctrine therefore to Catholic Christians 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.
-Commontorium 9:14

From this unity of Person it follows, by reason of a like mystery, that, since the flesh of the Word was born of an undefiled mother, God the Word Himself is most Catholicly believed, most impiously denied, to have been born of the Virgin; which being the case, God forbid that any one should seek to defraud Holy Mary of her prerogative of divine grace and her special glory. For by the singular gift of Him who is our Lord and God, and withal, her own son, she is to be confessed most truly and most blessedly — The mother of God Theotocos,
-Commontorium 15:9

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;
-Commontorium 20:25

“Keep the deposit” (1 Tim 20). What is The deposit? That which has been entrusted to you, not that which you have yourself devised: a matter not of wit, but of learning; not of private adoption, but of public tradition; a matter brought to you, not put forth by you, wherein you are bound to be not an author but a keeper, not a teacher but a disciple, not a leader but a follower. 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.
-Commontorium 22:27

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. For progress requires that the subject be enlarged n itself, alteration, that it be transformed into something else.
-Commontorium 23:28

“Therefore, as soon as the corruption of each mischievous error begins to break forth, and to defend itself by filching certain passages of Scripture, and expounding them fraudulently and deceitfully, immediately, the opinions of the ancients in the interpretation of the Canon are to be collected, whereby the novelty, and consequently the profaneness, whatever it may be, that arises, may both without any doubt be exposed, and without any tergiversation be condemned. But the opinions of those Fathers only are to be used for comparison, who living and teaching, holily, wisely, and with constancy, in the Catholic faith and communion, were counted worthy either to die in the faith of Christ, or to suffer death happily for Christ. Whom yet we are to believe in this condition, that that only is to be accounted indubitable, certain, established, which either all, or the more part, have supported and confirmed manifestly, frequently, persistently, in one and the same sense, forming, as it were, a consentient council of doctors, all receiving, holding, handing on the same doctrine. 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.
[73.] Lest any one perchance should rashly think the holy and Catholic consent of these blessed fathers to be despised, the Apostle says, in the First Epistle to the Corinthians, God has placed some in the Church, first Apostles, 1 Corinthians 12:27-28 of whom himself was one; secondly Prophets, such as Agabus, of whom we read in the Acts of the Apostles; Acts 11:28 then doctors, who are now called Homilists, Expositors, whom the same apostle sometimes calls also Prophets, because by them the mysteries of the Prophets are opened to the people. Whosoever, therefore, shall despise these, who had their appointment of God in His Church in their several times and places, when they are unanimous in Christ, in the interpretation of some one point of Catholic doctrine, despises not man, but God, from whose unity in the truth, lest any one should vary, the same Apostle earnestly protests, I beseech you, brethren, that you all speak the same thing, and that there be no divisions among you, but that you be perfectly joined together in the same mind and in the same judgment. 1 Corinthians 1:10 But if any one dissent from their unanimous decision, let him listen to the words of the same apostle, God is not the God of dissension but of peace; 1 Corinthians 14:33 that is, not of him who departs from the unity of consent, but of those who remain steadfast in the peace of consent: as, he continues, I teach in all Churches of the saints, that is, of Catholics, which churches are therefore churches of the saints, because they continue steadfast in the communion of the faith.”
-Commonitoria 28:72-73 [A.D. 434])

We said above, that it has always been the custom of Catholics, and still is, to prove the true faith in these two ways; first by the authority of the Divine Canon, and next by the tradition of the Catholic Church.
-Commontorium 29:76