How does a murderer cleanse and sanctify the water? How can darkness bless the oil? But if God is present in His sacraments to confirm His words by whomsoever the sacraments may be administered, then both the sacraments of God are everywhere valid, and evil men whom they profit not are everywhere perverse.
28. But what kind of argument is this, that "a heretic must be considered not to have baptism, because he has not the Church?" And it must be acknowledged that "when he is baptized, he is questioned about the Church." Just as though the same question about the Church were not put in baptism to him who within the Church renounces the world in word and not in deed. As therefore his false answer does not prevent what he receives from being baptism, so also the false reply of the other about the holy Church does not prevent what he receives from being baptism; and as the former, if he afterwards fulfill with truth what he promised in falsehood, does not receive a second baptism, but only an amended life, so also in the case of the latter, if he come afterwards to the Church about which he gave a false answer to the question put to him, thinking that he had it when he had it not, the Church herself which he did not possess is given him, but what he had received is not repeated. But I cannot tell why it should be, that while God can "sanctify the oil" in answer to the words which proceed out of the mouth of a murderer, "He yet cannot sanctify it on the altar reared by a heretic," unless it be that He who is not hindered by the false conversion of the heart of man within the Church is hindered by the false erection of some wood without from deigning to be present in His sacraments, though no falseness on the part of men can hinder Him. If, therefore, what is said in the gospel, that "God hears not sinners," extends so far that the sacraments cannot be celebrated by a sinner, how then does He hear a murderer praying, either over the water of baptism, or over the oil, or over the eucharist, or over the heads of those on whom his hand is laid? All which things are nevertheless done, and are valid, even at the hands of murderers, that is, at the hands of those who hate their brethren, even within, in the Church itself. Since "no one can give what he does not possess himself," how does a murderer give the Holy Spirit? And yet such an one even baptizes within the Church. It is God, therefore, that gives the Holy Spirit even when a man of this kind is baptizing.
Source: On Baptism, Against the Donatists (New Advent)