10 Wherefore, then, have ye severed yourselves? If there is any sense left in you, you must surely see that you can find no possible answer to these arguments. "We are not left," they say, "so utterly without resource, but that we can still answer, It is our will. 'Who are you that judgest another man's servant? To his own master he stands or falls.'" They do not understand that this was said to men who were wishing to judge, not of open facts, but of the hearts of other men. For how does the apostle himself come to say so much about the sins of schisms and heresies? Or how comes that verse in the Psalms, "If of a truth ye love justice, judge uprightly, O you sons of men?" But why does the Lord Himself say, "Judge not according to the appearance, but judge righteous judgment," if we may not judge any man? Lastly, why, in the case of those traditors, whom they have judged unrighteously, have they themselves ventured to pass any judgments at all on another man's servants? To their own master they were standing or falling. Or why, in the case of the recent followers of Maximianus, have they not hesitated to bring forward the judgment delivered with the infallible voice, as they aver, of a plenary Council, in such terms as to compare them with those first schismatics whom the earth swallowed up alive? And yet some of them, as they cannot deny, they either condemned though innocent, or received back again in their guilt. But when a truth is urged which they cannot gainsay, they mutter a truly wholesome murmuring: "It is our will: 'Who are you that judgest another man's servant? To his own master he stands or falls.'" But when a weak sheep is espied in the desert, and the pastor who should reclaim it to the fold is nowhere to be seen, then there is setting of teeth, and breaking of the weak neck: "You would be a good man, were you not a traditor. Consult the welfare of your soul; be a Christian." What unconscionable madness! When it is said to a Christian, "Be a Christian," what other lesson is taught, save a denial that he is a Christian? Was it not the same lesson which those persecutors of the Christians wished to teach, by resisting whom the crown of martyrdom was gained? Or must we even look on crime as lighter when committed with threatening of the sword than with treachery of the tongue?
11. Answer me this, you ravening wolves, who, seeking to be clad in sheep's clothing, think that the letters of the blessed Cyprian are in your favor. Did the sacrilege of schismatics defile Cyprian, or did it not? If it did, the Church perished from that instant, and there remained no source from which you might spring. If it did not, then by what offense on the part of others can the guiltless possibly be defiled, if the sacrilege of schism cannot defile them? Wherefore, then, have ye severed yourselves? Wherefore, while shunning the lighter offenses, which are inventions of your own, have ye committed the heaviest offense of all, the sacrilege of schism? Will ye now perchance confess that those men were no longer schismatics or heretics who had been baptized without the communion of the Church, or in some heresy or schism, because by coming over to the Church, and renouncing their former errors, they had ceased to be what formerly they were? How then was it, that though they were not baptized, their sins remained not on their heads? Was it that the baptism was Christ's, but that it could not profit them without the communion of the Church; yet when they came over, and, renouncing their past error, were received into the communion of the Church by the laying on of hands, then, being now rooted and founded in charity, without which all other things are profitless, they began to receive profit for the remission of sins and the sanctification of their lives from that sacrament, which, while without the pale of the Church, they possessed in vain?
12. Cease, then, to bring forward against us the authority of Cyprian in favor of repeating baptism, but cling with us to the example of Cyprian for the preservation of unity. For this question of baptism had not been as yet completely worked out, but yet the Church observed the most wholesome custom of correcting what was wrong, not repeating what was already given, even in the case of schismatics and heretics: she healed the wounded part, but did not meddle with what was whole. And this custom, coming, I suppose, from tradition (like many other things which are held to have been handed down under their actual sanction, because they are preserved throughout the whole Church, though they are not found either in their letters, or in the Councils of their successors),— this most wholesome custom, I say, according to the holy Cyprian, began to be what is called amended by his predecessor Agrippinus. But, according to the teaching which springs from a more careful investigation into the truth, which, after great doubt and fluctuation, was brought at last to the decision of a plenary Council, we ought to believe that it rather began to be corrupted than to receive correction at the hands of Agrippinus. Accordingly, when so great a question forced itself upon him, and it was difficult to decide the point, whether remission of sins and man's spiritual regeneration could take place among heretics or schismatics, and the authority of Agrippinus was there to guide him, with that of some few men who shared in his misapprehension of this question, having preferred attempting something new to maintaining a custom which they did not understand how to defend; under these circumstances considerations of probability forced themselves into the eyes of his soul, and barred the way to the thorough investigation of the truth.
Source: On Baptism, Against the Donatists (New Advent)