9 Wherefore, as the apostle said of the law, "The law is good, if a man use it lawfully," so we may fairly say of baptism, Baptism is good, if a man use it lawfully. And as they who used the law unlawfully could not in that case cause that it should not be in itself good, or make it null and void, so any one who uses baptism unlawfully, either because he lives in heresy, or because he lives the worst of lives, yet cannot cause that the baptism should be otherwise than good, or altogether null and void.
And so, when he is converted either to Catholic unity, or to a mode of living worthy of so great a sacrament, he begins to have not another and a lawful baptism, but that same baptism in a lawful manner. Nor does the remission of irrevocable sins follow on baptism, unless a man not only have lawful baptism, but have it lawfully; and yet it does not follow that if a man have it not lawfully, so that his sins are either not remitted, or, being remitted, are brought on him again, therefore the sacrament of baptism should be in the baptized person either bad or null and void.
For as Judas, to whom the Lord gave a morsel, gave a place within himself of the devil, not by receiving what was bad, but by receiving it badly, so each person, on receiving the sacrament of the Lord, does not cause that it is bad because he is bad himself, or that he has received nothing because he has not received it to salvation. For it was none the less the body of the Lord and the blood of the Lord, even in those to whom the apostle said, "He that eats unworthily, eats and drinks damnation to himself." Let the heretics therefore seek in the Catholic Church not what they have, but what they have not—that is, the end of the commandment, without which many holy things may be possessed, but they cannot profit.
"Now, the end of the commandment is charity out of a pure heart, and of a good conscience, and of faith unfeigned." Let them therefore hasten to the unity and truth of the Catholic Church, not that they may have the sacrament of washing, if they have been already bathed in it, although in heresy, but that they may have it to their health.
Source: On Baptism, Against the Donatists (New Advent)