With this initial post I begin with an excerpt from St. Vincent's "Commonitory." I've put the "Vincentian Canon" in bold type. Protestants and Catholics alike would benefit from a serious consideration of its implications.
A General Rule for distinguishing the Truth of the Catholic Faith from the Falsehood of Heretical Pravity.
[4.] 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.
[5.] 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.
What is to be done if one or more dissent from the rest.
[7.] 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.
[8.] But what, if in antiquity itself there be found error on the part of two or three men, or at any rate of a city or even of a province? Then it will be his care by all means, to prefer the decrees, if such there be, of an ancient General Council to the rashness and ignorance of a few. But what, if some error should spring up on which no such decree is found to bear? Then he must collate and consult and interrogate the opinions of the ancients, of those, namely, who, though living in divers times and places, yet continuing in the communion and faith of the one Catholic Church, stand forth acknowledged and approved authorities: and whatsoever he shall ascertain to have been held, written, taught, not by one or two of these only, but by all, equally, with one consent, openly, frequently, persistently, that he must understand that he himself also is to believe without any doubt or hesitation.