17 That therefore the baptism of John was not the same as the baptism of Christ, has, I think, been shown with sufficient clearness; and therefore no argument can be drawn from it that baptism should be repeated after heretics because it was repeated after John: since John was not a heretic, and could have a baptism, which, though granted by Christ, was yet not the very baptism of Christ, seeing that he had the love of Christ; while a heretic can have at once the baptism of Christ and the perversity of the devil, as another within the Church may have at once the baptism of Christ and the envy of the devil.
18. But it will be urged that baptism after a heretic is much more required, because John was not a heretic, and yet baptism was repeated after him. On this principle, a man may say, much more must we rebaptize after a drunkard, because John was sober, and yet baptism was repeated after him. And we shall have no answer to make to such a man, save that the baptism of Christ was given to those who were baptized by John, because they had it not; but where men have the baptism of Christ, no iniquity on their part can possibly effect that the baptism of Christ should fail to be in them.
19. It is not therefore true that "by baptizing first, the heretic obtains the right of baptism;" but because he did not baptize with his own baptism, and though he did not possess the right of baptizing, yet that which he gave is Christ's, and he who received it is Christ's. For many things are given wrongfully and yet they are not therefore said to be non-existent or not given at all. For neither does he who renounces the world in word only and not in deed receive baptism lawfully, and yet he does receive it. For both Cyprian records that there were such men in the Church in his day, and we ourselves experience and lament the fact.
20. But it is strange in what sense it can be said that "baptism and the Church cannot in any way be separated and detached from one another." For if baptism remains inseparably in him who is baptized, how can it be that he can be separated from the Church, and baptism cannot? But it is clear that baptism does remain inseparably in the baptized person; because into whatever depth of evil, and into whatever fearful whirlpool of sin the baptized person may fall, even to the ruin of apostasy, he yet is not bereft of his baptism. And therefore, if through repentance he returns, it is not given again, because it is judged that he could not have been bereft of it. But who can ever doubt that a baptized person can be separated from the Church? For hence all the heresies have proceeded which deceive by the use of Christian terms.
Source: On Baptism, Against the Donatists (New Advent)