Titus 3:11

For the heretic damns himself when he casts himself out of the Catholic church and under no compulsion leaves the gathering of the saints. He who separates himself from everyone by his own judgment shows what is merited from everyone. The heretic himself, I say, damns himself because, although all the wicked are cast out from the Christian assembly by the sentence of the bishop, the heretic departs himself, by the judgment of his own will, before anyone's subsequent wishes are expressed.

It is the same Paul who, in his epistle to the Galatians, counts "heresies" among "the sins of the flesh," who also intimates to Titus that "a man who is a heretic" must be "rejected after the first admonition." This is on the ground that "he that is perverted and commits sin as a self-condemned man." Indeed, in almost every epistle, when enjoining on us [the duty] of avoiding false doctrines, he sharply condemns heresies. Of these the practical effects are false doctrines, called in Greek heresis, a word used in the sense of that choice which a man makes when he either teaches them [to others] or takes up with them [for himself]. For this reason it is that he calls the heretic self-condemned, because he has himself chosen that in which he is condemned.

For the faith of the sacred Scripture sets forth that the Church is not without, nor can be separated nor divided against itself, but maintains the unity of an inseparable and undivided house; since it is written of the sacrament of the passover, and of the lamb, which Lamb designated Christ: "In one house shall it be eaten: ye shall not carry forth the flesh abroad out of the house." Which also we see expressed concerning Rahab, who herself also bore a type of the Church, who received the command which said, "Thou shalt bring thy father, and thy mother, and thy brethren, and all thy father's household unto thee into thine house; and whosoever shall go out of the doors of thine house into the street, his blood shall be upon him." In which mystery is declared, that they who will live, and escape from the destruction of the world, must be gathered together into one house alone, that is, into the Church; but whosoever of those thus collected together shall go out abroad, that is, if any one, although he may have obtained grace in the Church, shall depart and go out of the Church, that his blood shall be upon him; that is, that he himself must charge it upon himself that he perishes; which the Apostle Paul explains, teaching and enjoining that a heretic must be avoided, as perverse, and a sinner, and as condemned of himself. For that man will be guilty of his own ruin, who, not being cast out by the bishop, but of his own accord deserting from the Church is by heretical presumption condemned of himself.