29 But that I may not seem to be uttering these praises of the blessed martyr (which, indeed, are not his, but rather those of Him by whose grace he showed himself what he was), in order to escape the burden of proof, let us now bring forward from his letters the testimony by which the mouths of the Donatists may most of all be stopped. For they advance his authority before the unlearned, to show that in a manner they do well when they baptize afresh the faithful who come to them.
Too wretched are they— and, unless they correct themselves, even by themselves are they utterly condemned— who choose in the example set them by so great a man to imitate just that fault, which only did not injure him, because he walked with constant steps even to the end in that from which they have strayed who "have not known the way of peace." It is true that Christ's baptism is holy; and although it may exist among heretics or schismatics, yet it does not belong to the heresy or schism; and therefore even those who come from thence to the Catholic Church herself ought not to be baptized afresh.
Yet to err on this point is one thing; it is another thing that those who are straying from the peace of the Church, and have fallen headlong into the pit of schism, should go on to decide that any who join them ought to be baptized again. For the former is a speck on the brightness of a holy soul which abundance of charity would fain have covered; the latter is a stain in their nether foulness which the hatred of peace in their countenance ostentatiously brings to light. But the subject for our further consideration, relating to the authority of the blessed Cyprian, we will commence from a fresh beginning.
Source: On Baptism, Against the Donatists (New Advent)