2 Marcus of Mactaris said: "It is not to be wondered at if heretics, being enemies and opponents of the truth, claim to themselves what has been entrusted and vouchsafed to other men. What is marvellous is that some of us, traitors to the truth, uphold heretics and oppose Christians; therefore we decree that heretics should be baptized."
3. To him we answer: It is indeed much more to be wondered at, and deserving of expressions of great praise, that Cyprian and his colleagues had such love for unity that they continued in unity with those whom they considered to be traitors to the truth, without any apprehension of being polluted by them. For when Marcus said, "It is marvellous that some of us, traitors to the truth, uphold heretics and oppose Christians," it seemed natural that he should add, Therefore we decree that communion should not be held with them. This he did not say; but what he does say is, "Therefore we decree that heretics should be baptized," adhering to what the peaceful Cyprian had enjoined in the first instance, saying, "Judging no man, nor removing any from the right of communion if he entertain a different opinion." While, therefore, the Donatists calumniate us and call us traditors, I should be glad to know, supposing that any Jew or pagan were found, who, after reading the records of that Council should call both us and them, according to their own rules, traitors to the truth, how we should be able to make our joint defense so as to refute and wash away so grave a charge. They give the name of traditors to men whom they were never able in times past to convict of the offense, and whom they cannot now show to be involved in it, being themselves rather shown to be liable to the same charge. But what has this to do with us? What shall we say of them who, by their own showing, are unquestionably traitors? For if we, however falsely, are called traditors, because, as they allege, we took part in the same communion with traditors, we have all taken part with the traditors in question, seeing that in the time of the blessed Cyprian the party of Donatus had not yet separated itself from unity. For the delivery of the sacred books, from which they began to be called traditors, occurred somewhat more than forty years after his martyrdom. If, therefore, we are traditors, because we sprang from traditors, as they believe or pretend, we both of us derive our origin from those other traitors. For there is no room for saying that they did not communicate with these traitors, since they call them men of their own party. In the words of the Council which they are most forward to quote, "Some of us," it declares, "traitors to the truth, uphold heretics." To this is added the testimony of Cyprian, showing clearly that he remained in communion with them, when he says, "Judging no man, nor removing any from the right of communion if he entertain a different opinion." For those who entertained a different opinion were the very persons whom Marcus calls traitors to the truth because they upheld heretics, as he maintains, by receiving them into the Church without baptism. That it was, moreover, the custom that they should be so received, is testified both by Cyprian himself in many passages, and by some bishops in this Council. Whence it is evident that, if heretics have not baptism, the Church of Christ of those days was full of traitors, who upheld them by receiving them in this way. I would urge, therefore, that we plead our cause in common against the charge of treason which they cannot disavow, and therein our special case will be argued against the charge of delivering the books, which they could not prove against us. But let us argue the point as though they had convicted us; and what we shall answer jointly to those who urge against both of us the general treason of our forefathers, that we will answer to these men who urge against us that our forefathers gave up the sacred books. For as we were dead because our forefathers delivered up the books, which caused them to divide themselves from us, so both we and they themselves are dead through the treason of our forefathers, from whom both we and they are sprung. But since they say they live, they hold that that treason does not in any way affect them, therefore neither are we affected by the delivery of the books. And it should be observed that, according to them, the treason is indisputable: while, according to us, there is no truth either in the former charge of treason, because we say that heretics also may have the baptism of Christ; nor in the latter charge of delivering the books, because in that they were themselves beaten. They have therefore no reason for separating themselves by the wicked sin of schism, because, if our forefathers were not guilty of delivering up the books, as we say, there is no charge which can affect us at all; but if they were guilty of the sin, as these men say, then it is just as far from affecting us as the sin of those other traitors is from affecting either us or them. And hence, since there is no charge that can implicate us from the unrighteousness of our forefathers, the charge arising against them from their own schism is manifestly proved.
Source: On Baptism, Against the Donatists (New Advent)