33 By all these considerations it is proved that the sacrament of baptism is one thing, the conversion of the heart another; but that man's salvation is made complete through the two together. Nor are we to suppose that, if one of these be wanting, it necessarily follows that the other is wanting also; because the sacrament may exist in the infant without the conversion of the heart; and this was found to be possible without the sacrament in the case of the thief, God in either case filling up what was involuntarily wanting.
But when either of these requisites is wanting intentionally, then the man is responsible for the omission. And baptism may exist when the conversion of the heart is wanting; but, with respect to such conversion, it may indeed be found when baptism has not been received, but never when it has been despised. Nor can there be said in any way to be a turning of the heart to God when the sacrament of God is treated with contempt. Therefore we are right in censuring, anathematizing, abhorring, and abominating the perversity of heart shown by heretics; yet it does not follow that they have not the sacrament of the gospel, because they have not what makes it of avail.
Wherefore, when they come to the true faith, and by penitence seek remission of their sins, we are not flattering or deceiving them, when we instruct them by heavenly discipline for the kingdom of heaven, correcting and reforming in them their errors and perverseness, to the intent that we may by no means do violence to what is sound in them, nor, because of man's fault, declare that anything which he may have in him from God is either valueless or faulty.
Source: On Baptism, Against the Donatists (New Advent)