“Also in the Catholic Church itself we take great care that we hold that which has been believed everywhere, always, by all. For that is truly and properly Catholic, as the very force and meaning of the word shows, which comprehends everything almost universally. And we shall observe this rule 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 plain that our ancestors and fathers proclaimed; consent if in antiquity itself we eagerly follow the definitions and beliefs of all, or certainly nearly all, priests and doctors alike.”
“What, then, will the Catholic Christian do if any part of the Church has cut itself off from the communion of the Universal Faith? What surely but prefer the soundness of the whole body to a pestilent and corrupt member?
“What if some novel contagion seeks to infect the whole Church, and not merely a small portion of it? Then he will take care to cling to antiquity, which cannot now be led astray by any novel deceit.
“What if in antiquity itself error be detected on the part of two or three men, or perhaps of a city, or even of a province? Then he will look to it that he prefer the decrees of an ancient General Council, if such there be, to the rashness and ignorance of a few.
“But what if some error spring up concerning which nothing of this kind is to be found? Then he must take pains to find out and compare the opinions of the ancients, provided, of course, that such remained in the communion and faith of the One Catholic Church, although they lived in different times and places, conspicuous and approved teachers; and whatever he shall find to have been held, written and taught, not by one or two only, but by all equally and with one consent, openly, frequently and persistently, that he must understand is to be believed by himself also without the slightest hesitation.“
— St. Vincent of Lerins, “The Commonitory,“ tr. by T. Herbert Bindley (London: SPCK, 1914), book 1, chapter 2, no. 6-8, pp. 26-28. Emphasis added.
From http://www.strobertbellarmine.net/lerins.html, an excellent resource on Sedevacantism and on Thomistic teaching.