88 Also another Aurelius of Chullabi said: "The Apostle John has laid down in his epistle the following precept: 'If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed: for he that bids him God speed is partaker of his evil deeds.' How can such men be admitted without consideration into the house of God, who are forbidden to be admitted into our private house? Or how can we hold communion with them without the baptism of Christ, when, if we only so much as bid them God speed, we are partakers of their evil deeds?"
89. In respect of this testimony of John there is no need of further disputation, since it has no reference at all to the question of baptism, which we are at present discussing. For he says, "If any come unto you, and bring not the doctrine of Christ." But heretics leaving the doctrine of their error are converted to the doctrine of Christ, that they may be incorporated with the Church, and may begin to belong to the members of that Dove whose sacrament they previously had; and therefore what previously they lacked belonging to it is given to them, that is to say, peace and charity out of a pure heart, and of a good conscience, and of faith unfeigned. But what they previously had belonging to the Dove is acknowledged, and received without any depreciation; just as in the adulteress God recognises His gifts, even when she is following her lovers; because when after her fornication is corrected she is turned again to chastity, those gifts are not laid to her charge, but she herself is corrected. But just as Cyprian might have defended himself if this testimony of John had been cast in his teeth while he was holding communion with men like these, so let those against whom it is spoken make their own defense. For to the question before us, as I said before, it has no reference at all. For John says that we are not to bid God speed to men of strange doctrine; but Paul the apostle says, with even greater vehemence, "If any man that is called a brother be covetous, or a drunkard," or anything of the sort, with such an one no not to eat; and yet Cyprian used to admit to fellowship, not with his private table, but with the altar of God, his colleagues who were usurers, and treacherous, and fraudulent, and robbers. But in what manner this may be defended has been sufficiently set forth in other books already.
Source: On Baptism, Against the Donatists (New Advent)