46 You will say, “Why then do you seek us? Why do you receive those whom you call heretics?” Mark how simple and short is my reply. We seek you because you are lost, that we may rejoice over you when found, as over you while lost we grieved. Again we call you heretics; but the name applies to you only up to the time of your being turned to the peace of the Catholic Church, and extricated from the errors by which you have been ensnared. For when you pass over to us, you entirely abandon the position you formerly occupied, so that, as heretics no longer, you pass over to us. You will say, “Then baptize me.” I would, if you were not already baptized, or if you had received the baptism of Donatus, or of Rogatus only, and not of Christ. It is not the Christian sacraments, but the crime of schism, which makes you a heretic. The evil which has proceeded from yourself is not a reason for our denying the good that is permanent in you, but which you possess to your own harm if you have it not in that Church from which proceeds its power to do good. For from the Catholic Church are all the sacraments of the Lord, which you hold and administer in the same way as they were held and administered even before you went forth from her. The fact, however, that you are no longer in that Church from which proceeded the sacraments which you have, does not make it the less true that you still have them. We therefore do not change in you that wherein you are at one with ourselves, for in many things you are at one with us; and of such it is said, “For in many things they were with me:” but we correct those things in which you are not with us, and we wish you to receive those things which you have not where you now are. You are at one with us in baptism, in creed, and in the other sacraments of the Lord. But in the spirit of unity and bond of peace, in a word, in the Catholic Church itself, you are not with us. If you receive these things, the others which you already have will then not begin to be yours, but begin to be of use to you. We do not therefore, as you think, receive your men of your party as still belonging to you, but in the act of receiving them we incorporate with ourselves those who forsake you that they may be received by us; and in order that they may belong to us, their first step is to renounce their connection with you. Nor do we compel into union with us those who industriously serve an error which we abhor; but our reason for wishing those men to be united to us is, that they may no longer be worthy of our abhorrence.
47. But you will say, “The Apostle Paul baptized after John.” Did he then baptize after a heretic? If you do presume to call that friend of the Bridegroom a heretic, and to say that he was not in the unity of the Church, I beg that you will put this in writing. But if you believe that it would be the height of folly to think or to say so, it remains for your own wisdom to resolve the question why the Apostle Paul baptized after John. For if he baptized after one who was his equal, you ought all to baptize after one another. If after one who was greater than himself, you ought to baptize after Rogatus; if after one who was less than himself, Rogatus ought to have baptized after you those whom you, as a presbyter, had baptized. If, however, the baptism which is now administered is in all cases of equal value to those who receive it, however unequal in merit the persons may be by whom it is administered, because it is the baptism of Christ, not of those who administer the right, I think you must already perceive that Paul administered the baptism of Christ to certain persons because they had received the baptism of John only, and not of Christ; for it is expressly called the baptism of John, as the Divine Scripture bears witness in many passages, and as the Lord Himself calls it, saying: “The baptism of John, whence was it? From heaven, or of men?” But the baptism which Peter administered was the baptism, not of Peter, but of Christ; that which Paul administered was the baptism, not of Paul, but of Christ; that which was administered by those who, in the apostle's time, preached Christ not sincerely, but of contention, was not their own, but the baptism of Christ; and that which was administered by those who, in Cyprian's time, either by artful dishonesty obtained their possessions, or by usury, at exorbitant interest, increased them, was not their own baptism, but the baptism of Christ. And because it was of Christ, therefore, although there was very great disparity in the persons by whom it was administered, it was equally useful to those by whom it was received. For if the excellency of baptism in each case is according to the excellency of the person by whom one is baptized, it was wrong in the apostle to give thanks that he had baptized none of the Corinthians, but Crispus, and Gaius, and the house of Stephanas; for the baptism of the converts in Corinth, if administered by himself, would have been so much more excellent as Paul himself was more excellent than other men. Lastly, when he says, “I have planted, and Apollos watered,” he seems to intimate that he had preached the gospel, and that Apollos had baptized. Is Apollos better than John? Why then did he, who baptized after John, not baptize after Apollos? Surely because, in the one case, the baptism, by whomsoever administered, was the baptism of Christ; and in the other case, by whomsoever administered, it was, although preparing the way for Christ, only the baptism of John.
Source: Letters (New Advent)