77 Also another Felix of Ammacura said: "I too, following the authority of the holy Scriptures, give my judgment that heretics should be baptized, and with them those also who maintain that they have been baptized among schismatics. For if, according to the warning of Christ, our fountain is sealed to ourselves, let all the enemies of our Church understand that it cannot belong to others; nor can He who is the Shepherd of our flock give the water unto salvation to two different peoples. And therefore it is clear that neither heretics nor schismatics can receive anything heavenly, who dare to accept from men that are sinners and aliens from the Church. When the giver has no ground to stand upon, surely neither can the receiver derive any profit."
78. To him we answer, that the holy Scriptures nowhere have enjoined that heretics baptized among heretics should be baptized afresh, but that they have shown in many places that all are aliens from the Church who are not on the rock, nor belong to the members of the dove, and yet that they baptize and are baptized and have the sacrament of salvation without salvation. But how our fountain is like the fountain of Paradise, in that, like it, it flows forth even beyond the bounds of Paradise, has been sufficiently set forth above; and that "He who is the Shepherd of our flock cannot give the water unto salvation to two different peoples," that is, to one that is His own, and to another that is alien, I fully agree in admitting. But does it follow that because the water is not unto salvation it is not the identical water? For the water of the deluge was for salvation unto those who were placed within the ark, but it brought death to those without, and yet it was the same water. And many aliens, that is to say, envious persons, whom Cyprian declares and proves from Scripture to be of the party of the devil, seem as it were to be within, and yet, if they were not without the ark, they would not perish by water. For such men are slain by baptism, as the sweet savor of Christ was unto death to those of whom the apostle speaks. Why then do not either heretics or schismatics receive anything heavenly, just as thorns or tares, like those who were without the ark received indeed the rain from the floods of heaven, but to destruction, not to salvation? And so I do not take the pains to refute what he said in conclusion: "When the giver has no ground to stand upon, surely neither can the receiver derive any profit," since we also say that it does not profit the receivers while they receive it in heresy, consenting with the heretics; and therefore they come to Catholic peace and unity, not that they may receive baptism, but that what they had received may begin to profit them.
Source: On Baptism, Against the Donatists (New Advent)