20 Since the Catholic Church, both in the time of the blessed Cyprian and in the older time before him, contained within her bosom either some that were rebaptized or some that were unbaptized, either the one section or the other must have won their salvation only by the force of simple unity. For if those who came over from the heretics were not baptized, as Cyprian asserts, they were not rightly admitted into the Church; and yet he himself did not despair of their obtaining pardon from the mercy of God in virtue of the unity of the Church.
So again, if they were already baptized, it was not right to rebaptize them. What, therefore, was there to aid the other section, save the same charity that delighted in unity, so that what was hidden from man's weakness, in the consideration of the sacrament, might not be reckoned, by the mercy of God, as a fault in those who were lovers of peace? Why, then, while you fear those whom you have rebaptized, do ye grudge yourselves and them the entrance to salvation? There was at one time a doubt upon the subject of baptism; those who held different opinions yet remained in unity.
In course of time, owing to the certain discovery of the truth, that doubt was taken away. The question which, unsolved, did not frighten Cyprian into separation from the Church, invites you, now that it is solved, to return once more within the fold. Come to the Catholic Church in its agreement, which Cyprian did not desert while yet disturbed with doubt; or if now you are dissatisfied with the example of Cyprian, who held communion with those who were received with the baptism of heretics, declaring openly that we should "neither judge any one, nor deprive any one of the right of communion if he differ from us," whither are you going, you wretched men?
What are you doing? You are bound to fly even from yourselves, because you have advanced beyond the position where he abode. But if neither his own sins nor those of others could stand in his way, on account of the abundance of his charity and his love of brotherly kindness and the bond of peace, do you return to us, where you will find much less hindrance in the way of either us or you from the fictions which your party have invented.
Source: On Baptism, Against the Donatists (New Advent)