49 Privatianus of Sufetula said: "He who says that heretics have the power of baptizing should first say who it was that founded heresy. For if heresy is of God, it may have the divine favor; but if it be not of God, how can it either have or confer on any one the grace of God?"
50. This man may thus be answered word for word: He who says that malicious and envious persons have the power of baptizing, should first say who was the founder of malice and envy. For if malice and envy are of God, they may have the divine favor; but if they are not of God, how can they either have or confer on any one the grace of God? But as these words are in the same way most manifestly false, so are also those which these were uttered to confute. For the malicious and envious baptize, as even Cyprian himself allows, because he bears testimony that they also are within. So therefore even heretics may baptize, because baptism is the sacrament of Christ; but envy and heresy are the works of the devil. Yet though a man possesses them, he does not thereby cause that if he have the sacrament of Christ, it also should itself be reckoned in the number of the devil's works.
Source: On Baptism, Against the Donatists (New Advent)