25 "Can the power of baptism," says Cyprian, "be greater or better than confession? Than martyrdom? That a man should confess Christ before men, and be baptized in his own blood? And yet," he goes on to say, "neither does this baptism profit the heretic, even though for confessing Christ he be put to death outside the Church." This is most true; for, by being put to death outside the Church, he is proved not to have had charity, of which the apostle says, "Though I give my body to be burned, and have not charity, it profits me nothing." But if martyrdom is of no avail for this reason, because it has not charity, neither does it profit those who, as Paul says, and Cyprian further sets forth, are living within the Church without charity in envy and malice; and yet they can both receive and transmit true baptism.
"Salvation," he says, "is not without the Church." Who says that it is? And therefore, whatever men have that belongs to the Church, it profits them nothing towards salvation outside the Church. But it is one thing not to have, another to have so as to be of no use. He who has not must be baptized that he may have; but he who has to no avail must be corrected, that what he has may profit him. Nor is the water in the baptism of heretics "adulterous," because neither is the creature itself which God made evil, nor is fault to be found with the words of the gospel in the mouths of any who are astray; but the fault is theirs in whom there is an adulterous spirit, even though it may receive the adornment of the sacrament from a lawful spouse.
Baptism therefore can "be common to us, and the heretics," just as the gospel can be common to us, whatever difference there may be between our faith and their error—whether they think otherwise than the truth about the Father, or the Son, or the Holy Spirit; or, being cut away from unity, do not gather with Christ, but scatter abroad, — seeing that the sacrament of baptism can be common to us, if we are the wheat of the Lord, with the covetous within the Church, and with robbers, and drunkards, and other pestilent persons of the same sort, of whom it is said, "They shall not inherit the kingdom of God," and yet the vices by which they are separated from the kingdom of God are not shared by us.
Source: On Baptism, Against the Donatists (New Advent)