This Catholic Church, you see, spread firmly far and wide throughout the whole world, makes use, for the advancement of her members and for their own correction, of all who go astray, when they are prepared to wake up. I mean, the pagan gentiles provide her with the material for her work, the heretics help her to refine and test her doctrine, the schismatics enable her to demonstrate her stability, the Jews provide her with a contrast to her beauty. So then, some she invites into her house, others she turns away from the door, others she leaves alone, others she excels; to all, however, she offers the possibility of sharing in the grace of God, whether they are still to be formed in it, or in it to be reformed, or to be gathered in again, or to be admitted for the first time.
So, then, we must repudiate all those who neither philosophize about sacred matters nor attach sacred rites to philosophy, as well as those who by their distorted ideas or by their proud and jealous rivalry have deviated from the rule and communion of the Catholic Church; that group also which has refused to accept what illuminates their scriptures and accords grace to a new people, which is called the New Testament; all of these I have summarized above as briefly as I could. Instead, we must hold on to the Chrisitian religion and to communion with that Church which is Catholic and is called Catholic not only by its members but also by all its enemies. Willy-nilly, after all, heretics themselves, and the alumni of schisms, when talking not among themselves but with outsiders, have no other name for the Catholic Church but “Catholic.” I mean, they would not be understood unless they distinguished it by the name which is given it by the whole world.
De vera religione 6, 10; 7, 12