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

Chapter 5.—5.  Wherefore, even if heretics should be truly anxious to correct their error and come to the Church, for the very reason that they believed that they had no baptism unless they received it in the Church, even under these circumstances we should not be bound to yield to their desire for the repetition of baptism; but rather they should be taught, on the one hand, that baptism, though perfect in itself, could in no way profit their perversity if they would not submit to be corrected; and, on the other hand, that the perfection of baptism could not be impaired by their perversity, while refusing to be corrected:  and again, that no further perfection is added to baptism in them because they are submitting to correction; but that, while they themselves are quitting their iniquity, that which was before within them to their destruction is now beginning to be of profit for salvation.  For, learning this, they will both recognize the need of salvation in Catholic unity, and will cease to claim as their own what is really Christ’s, and will not confound the sacrament of truth, although existing in themselves, with their own individual error.

6.  To this we may add a further reason, that men, by a sort of hidden inspiration from heaven, shrink from any one who for the second time receives baptism which he had already received in any quarter whatsoever, insomuch that the very heretics themselves, when their arguments start with that subject, rub their forehead in perplexity, and almost all their laity, even those who have grown old in their body, and have conceived an obstinate animosity against the Catholic Church, confess that this one point in their system displeases them; and many who, for the sake of gaining some secular advantage, or avoiding some disadvantage, wish to secede to them, strive with many secret efforts that they may have granted to them, as a peculiar and individual privilege, that they should not be rebaptized; and some, who are led to place credence in their other vain delusions and false accusations against the Catholic Church, are recalled to unity by this one consideration, that they are unwilling to associate with them lest they should be compelled to be rebaptized.  And the Donatists, through fear of this feeling, which has so thorough possession of all men’s hearts, have consented to acknowledge the baptism which was conferred among the followers of Maximianus, whom they had condemned, and so to cut short their own tongues p. 466 and close their mouths, in preference to baptizing again so many men of the people of Musti, and Assuræ, and other districts, whom they received with Felicianus and Prætextatus, and the others who had been condemned by them and afterwards returned to them.