3 Again, if any one not having charity, and walking in the abandoned paths of a most wicked life, seems to be within while he really is without, and at the same time does not seek for the repetition of baptism even in the case of heretics, it in no wise helps his barrenness, because he is not rendered fruitful with his own fruit, but laden with that of others. But it is possible that some one may flourish in the root of charity, and may be most rightly minded in the point in which Cyprian was otherwise minded, and yet there may be more that is fruitful in Cyprian than in him, more that requires purging in him than in Cyprian. Not only, therefore, do we not compare bad Catholics with the blessed Cyprian, but even good Catholics we do not hastily pronounce to be on an equality with him whom our pious mother Church counts among the few rare men of surpassing excellence and grace, although these others may recognize the baptism of Christ even among heretics, while he thought otherwise; so that, by the instance of Cyprian, who saw one point less clearly, and yet remained most firm in the unity of the Church, it might be shown more clearly to heretics what a sacrilegious crime it was to break the bond of peace. For neither were the blind Pharisees, although they sometimes enjoined what was right to be done, to be compared to the Apostle Peter, though he at times enjoined what was not right. But not only is their dryness not to be compared to his greenness, but even the fruit of others may not be deemed equal to his fertility. For no one now compels the Gentiles to judaize, and yet no one now in the Church, however great his progress in goodness, may be compared with the apostleship of Peter. Wherefore, while rendering due reverence, and paying, so far as I can, the fitting honor to the peaceful bishop and glorious martyr Cyprian, I yet venture to say that his view concerning the baptism of schismatics and heretics was contrary to that which was afterwards brought to light by a decision, not of mine, but of the whole Church, confirmed and strengthened by the authority of a plenary Council: just as, while paying the reverence he deserves to Peter, the first of the apostles and most eminent of martyrs, I yet venture to say that he did not do right in compelling the Gentiles to judaize; for this also, I say, not of my own teaching, but according to the wholesome doctrine of the Apostle Paul, retained and preserved through out the whole Church.
4. Therefore, in discussing the opinion of Cyprian, though myself of far inferior merit to Cyprian, I say that good and bad alike can have, can give, can receive the sacrament of baptism—the good, indeed, to their health and profit; the bad to their destruction and ruin—while the sacrament itself is of equal perfectness in both of them; and that it is of no consequence to its equal perfectness in all, how much worse the man may be that has it among the bad, just as it makes no difference how much better he may be that has it among the good. And accordingly it makes no difference either how much worse he may be that confers it, as it makes no difference how much better he may be; and so it makes no difference how much worse he may be that receives it, as it makes no difference how much better he may be. For the sacrament is equally holy, in virtue of its own excellence, both in those who are unequally just, and in those who are unequally unjust.
Source: On Baptism, Against the Donatists (New Advent)