18 What shall we say of what is also wonderful, that he who carefully observes may find that it is possible that certain persons, without violating Christian charity, may yet teach what is useless, as Peter wished to compel the Gentiles to observe Jewish customs, as Cyprian himself would force heretics to be baptized anew? Whence the apostle says to such good members, who are rooted in charity, and yet walk not rightly in some points, "If in anything ye be otherwise minded, God shall reveal even this unto you;" and that some again, though devoid of charity, may teach something wholesome?
Of whom the Lord says, "The scribes and the Pharisees sit in Moses' seat: all therefore whatsoever they bid you observe, that observe and do; but do not ye after their works: for they say and do not." Whence the apostle also says of those envious and malicious ones who yet preach salvation through Christ, "Whether in pretense, or in truth, let Christ be preached." Wherefore, both within and without, the waywardness of man is to be corrected, but the divine sacraments and utterances are not to be attributed to men.
He is not, therefore, a "patron of heretics" who refuses to attribute to them what he knows not to belong to them, even though it be found among them. We do not grant baptism to be theirs; but we recognize His baptism of whom it is said, "The same is He which baptizes," wheresoever we find it. But if "the treacherous and blasphemous man" continue in his treachery and blasphemy, he receives no "remission of sins either without" or within the Church; or if, by the power of the sacrament, he receives it for the moment, the same force operates both without and within, as the power of the name of Christ used to work the expulsion of devils even without the Church.
Source: On Baptism, Against the Donatists (New Advent)