The Vincentian Canon:
"Now in the Catholic Church itself we take the greatest care to hold that which has been believed everywhere, always and by all. That is truly and properly 'Catholic,' as is shown by the very force and meaning of the word, which comprehends everything almost universally. We shall hold to this rule if we follow universality [i.e. oecumenicity], antiquity, and consent. We shall follow universality if we acknowledge 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 clear that our ancestors and fathers proclaimed; consent, if in antiquity itself we keep following the definitions and opinions of all, or certainly nearly all, bishops and doctors alike.
(4) What then will the Catholic Christian do, if a small part of the Church has cut itself off from the communion of the universal Faith? The answer is sure. He will prefer the healthiness of the whole body to the morbid and corrupt limb. But what if some novel contagion try to infect the whole Church, and not merely a tiny part of it? Then he will take care to cleave to antiquity, which cannot now be led astray by any deceit of novelty. What if in antiquity itself two or three men, or it may be a city, or even a whole province be detected in error? Then he will take the greatest care to prefer the decrees of the ancient General Councils, if there are such, to the irresponsible ignorance of a few men. But what if some error arises regarding which nothing of this sort is to be found? Then he must do his best to compare the opinions of the Fathers and inquire their meaning, provided always that, though they belonged to diverse times and places, they yet continued in the faith and communion of the one Catholic Church; and let them be teachers approved and outstanding. And whatever he shall find to have been held, approved and taught, not by one or two only but by all equally and with one consent, openly, frequently, and persistently, let him take this as to be held by him without the slightest hesitation."
St. Vincent of Lerins, ca. 435