28 Saturninus of Victoriana said: "If heretics may baptize, they are excused and defended in doing unlawful things; nor do I see why either Christ called them His adversaries, or the apostle called them antichrists."
29. To him we answer: We say that heretics have no authority to baptize in the same sense in which we say that defrauders have no authority to baptize. For not only to the heretic, but to the sinner, God says, "What have you to do to declare my statutes, or that you should take my covenant in your mouth?" To the same person He assuredly says, "When you saw a thief, then you consented with him." How much worse, therefore, are those who did not consent with thieves, but themselves were wont to plunder farms with treacherous deceits? Yet Cyprian did not consent with them, though he did tolerate them in the grain-field of the Catholic Church, lest the wheat should be rooted out together with it. And yet at the same time the baptism which they themselves conferred was the very selfsame baptism, because it was not of them, but of Christ. As therefore they, although the baptism of Christ be recognized in them, were yet not excused and defended in doing unlawful things, and Christ rightly called those His adversaries who were destined, by persevering in such things, to hear the doom, "Depart from me, you that work iniquity," whence also they are called antichrists, because they are contrary to Christ while they live in opposition to His words, so likewise is it the case with heretics.
Source: On Baptism, Against the Donatists (New Advent)