15 They ask also, "Whether sins are remitted in baptism in the party of Donatus:" so that, if we say that they are remitted, they may answer, then the Holy Spirit is there; for when by the breathing of our Lord the Holy Spirit was given to the disciples, He then went on to say, "Baptize all nations in the name of the Father, and of the Son, and of the Holy Ghost." Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained." And if it is so, they say, then our communion is the Church of Christ; for the Holy Spirit does not work the remission of sins except in the Church. And if our communion is the Church of Christ, then your communion is not the Church of Christ. For that is one, wherever it is, of which it is said, "My dove is but one; she is the only one of her mother;" nor can there be just so many churches as there are schisms. But if we should say that sins are not there remitted, then, say they, there is no true baptism there; and therefore ought you to baptize those whom you receive from us. And since you do not do this, you confess that you are not in the Church of Christ.
16. To these we reply, following the Scriptures, by asking them to answer themselves what they ask of us. For I beg them to tell us whether there is any remission of sins where there is not charity; for sins are the darkness of the soul. For we find St. John saying, "He that hates his brother is still in darkness." But none would create schisms, if they were not blinded by hatred of their brethren. If, therefore, we say that sins are not remitted there, how is he regenerate who is baptized among them? And what is regeneration in baptism, except the being renovated from the corruption of the old man? And how can he be so renovated whose past sins are not remitted? But if he be not regenerate, neither does he put on Christ; from which it seems to follow that he ought to be baptized again. For the apostle says, "For as many of you as have been baptized into Christ have put on Christ;" and if he has not so put on Christ, neither should he be considered to have been baptized in Christ. Further, since we say that he has been baptized in Christ, we confess that he has put on Christ; and if we confess this, we confess that he is regenerate. And if this be so, how does St. John say, "He that hates his brother remains still in darkness," if remission of his sins has already taken place? Can it be that schism does not involve hatred of one's brethren? Who will maintain this, when both the origin of, and perseverance in schism consists in nothing else save hatred of the brethren?
17. They think that they solve this question when they say: "There is then no remission of sins in schism, and therefore no creation of the new man by regeneration, and accordingly neither is there the baptism of Christ." But since we confess that the baptism of Christ exists in schism, we propose this question to them for solution: Was Simon Magus endued with the true baptism of Christ? They will answer, Yes; being compelled to do so by the authority of holy Scripture. I ask them whether they confess that he received remission of his sins. They will certainly acknowledge it. So I ask why Peter said to him that he had no part in the lot of the saints. Because, they say, he sinned afterwards, wishing to buy with money the gift of God, which he believed the apostles were able to sell.
Source: On Baptism, Against the Donatists (New Advent)