Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Donatist Controversy.: Chapter 5

Chapter 5.—7.  Wherefore all bad men are separated in the spirit from the good; but if they are separated in the body also by a manifest dissension, they are made yet worse.  But, as it has been said, it makes no difference to the holiness of baptism how much worse the man may be that has it, or how much worse he that confers it:  yet he that is separated may confer it, as he that is separated may have it; but as he has it to destruction, so he may confer it to destruction.  But he on whom he confers it may receive it to his soul’s health, if he, on his part, receive it not in separation; as it has happened to many that, in a catholic spirit, and with heart not alienated from the unity of peace, they have, under some pressure of impending death, turned hastily to some heretic and received from him the baptism of Christ without any share in his perversity, so that, whether dying or restored to life, they by no means remain in communion with those to whom they never passed in heart.  But if the recipient himself has received the baptism in separation, he receives it so much the more to his destruction, in proportion to the greatness of the good which he has not received well; and it tends the more to his destruction in his separation, as it would avail the more to the salvation of one in unity.  And so, if, reforming himself from his perverseness and turning from his separation, he should come to the Catholic peace, his sins are remitted through the bond of peace and the same baptism under which his sins were retained through the sacrilege of separation, because that is always holy both in the just and the unjust, which is neither increased by the righteousness nor diminished by the unrighteousness of any man.

8.  This being the case, what bearing has it on so clear a truth, that many of his fellow-bishops agreed with Cyprian in that opinion, and advanced their own several opinions on the same side, except that his charity towards the unity of Christ might become more and more conspicuous?  For if he had been the only one to hold that opinion, with no one to agree with him, he might have been thought, in remaining, to have shrunk from the sin of schism, because he found no companions in his error; but when so many agreed with him, he showed, by remaining in unity with the rest who thought differently from him, that he preserved the most sacred bond of universal catholicity, not from any fear of isolation, but from the love of peace.  Wherefore it might indeed seem now to be superfluous to consider the several opinions of the other bishops also in that Council; but since those who are slow in heart think that no answer has been made at all, if to any passage in any discourse the answer which might be brought to bear on the spot be given not there but somewhere else, it is better that by reading much they should be polished into sharpness, than that by understanding little they should have room left for complaining that the argument has not been fairly conducted.