Recommended Book: First Epistle of Clement to the Corinthians - Kindle Edition (get Kindle app for phone) (paid link)
This being the case, 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; but that whatsoever new and unheard of doctrine he shall find to have been furtively introduced by some one or another, besides that of all, or contrary to that of all the saints, this, he will understand, does not pertain to religion, but is permitted as a trial, being instructed especially by the words of the blessed Apostle Paul, who writes thus in his first Epistle to the Corinthians, `There must needs be heresies, that they who are approved may be made manifest among you:' as though he should say, This is the reason why the authors of Heresies are not forthwith rooted up by God, namely, that they who are approved may be made manifest that is, that it may be apparent of each individual, how tenacious and faithful and steadfast he is in his love of the Catholic faith.
Vincent of Lerins [d. ante A.D. 450]