Excerpt from "The Notebooks" by Saint Vincent of Lerins

[2,1] 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.

[2,2] 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 ecclesiatical interpretation be joined to it?" Because, quite plainly, Sacred Scripture, by reason of ts 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. (Now for the historical characters) Novatian explains a passage in one way, Sabellius in another, Donatus in another; Arius, Eunomius, Macedonius in another; Photinus, Apollinarius, 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.

[2,3] In the Catholic Church herself every care must be taken that we hold fast to that which has been believed everywhere, always, and by all. 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 universally. This general rule will be correctly applied if we pursue universality, antiquity, and agreement. And we follow universality in this way, if we confess this one faith to be true, which is confessed by the whole Church throughout the whole world; antiquity, however, if we can 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.

Saint Vincent was a priest and monk at Lerins, the island monastery near Nice. He wrote three works and was martyred in A.D. 450.

Source: The Faith of the Early Fathers, Volume 3, by William A. Jurgens