6 We do not, therefore, "acknowledge the baptism of heretics," when we refuse to baptize after them; but because we acknowledge the ordinance to be of Christ even among evil men, whether openly separated from us, or secretly severed while within our body, we receive it with due respect, having corrected those who were wrong in the points wherein they went astray. However as I seem to be hard pressed when it is said to me, "Does then a heretic confer remission of sins?" so I in turn press hard when I say, Does then he who violates the commands of Heaven, the avaricious man, the robber, the usurer, the envious man, does he who renounces the world in words and not in deeds, confer such remission? If you mean by the force of God's sacrament, then both the one and the other; if by his own merit, neither of them. For that sacrament, even in the hands of wicked men, is known to be of Christ; but neither the one nor the other of these men is found in the body of the one uncorrupt, holy, chaste dove, which has neither spot nor wrinkle. And just as baptism is of no profit to the man who renounces the world in words and not in deeds, so it is of no profit to him who is baptized in heresy or schism; but each of them, when he amends his ways, begins to receive profit from that which before was not profitable, but was yet already in him.
7. "He therefore that is baptized in heresy does not become the temple of God; but does it therefore follow that he is not to be considered as baptized? For neither does the avaricious man, baptized within the Church, become the temple of God unless he depart from his avarice; for they who become the temple of God certainly inherit the kingdom of God. But the apostle says, among many other things, "Neither the covetous, nor extortioners, shall inherit the kingdom of God." For in another place the same apostle compares covetousness to the worship of idols: "Nor covetous man," he says, "who is an idolater;" which meaning the same Cyprian has so far extended in a letter to Antonianus, that he did not hesitate to compare the sin of covetousness with that of men who in time of persecution had declared in writing that they would offer incense. The man, then, who is baptized in heresy in the name of the Holy Trinity, yet does not become the temple of God unless he abandons his heresy, just as the covetous man who has been baptized in the same name does not become the temple of God unless he abandons his covetousness, which is idolatry. For this, too, the same apostle says: "What agreement has the temple of God with idols?" Let it not, then, be asked of us "of what God he is made the temple" when we say that he is not made the temple of God at all. Yet he is not therefore unbaptized, nor does his foul error cause that what he has received, consecrated in the words of the gospel, should not be the holy sacrament; just as the other man's covetousness (which is idolatry) and great uncleanness cannot prevent what he receives from being holy baptism, even though he be baptized with the same words of the gospel by another man covetous like himself.
Source: On Baptism, Against the Donatists (New Advent)