71 Libosus of Vaga said: "The Lord says in the gospel, 'I am the truth;' He did not say, I am custom. Therefore, when the truth is made manifest, let custom yield to truth; so that, if even in time past any one did not baptize heretics in the Church, he may now begin to baptize them."
72. Here he has in no way tried to show how that is the truth to which he says that custom ought to yield. But it is of more importance that he helps us against those who have separated themselves from unity, by confessing that the custom existed, than that he thinks it ought to yield to a truth which he does not show. For the custom is of such a nature, that if it admitted sacrilegious men to the altar of Christ without the cleansing of baptism, and polluted none of the good men who remained in unity, then all who have cut themselves off from the same unity, in which they could not be polluted by the contagion of any evil persons whatsoever, have separated themselves without reason, and have committed the manifest sacrilege of schism. But if all perished in pollution through that custom, from what cavern do they issue without the original truth, and with all the cunning of calumny? If, however, the custom was a right one by which heretics were thus received, let them abandon their madness, let them confess their error; let them come to the Catholic Church, not that they may be bathed again with the sacrament of baptism, but that they may be cured from the wound of severance.
Source: On Baptism, Against the Donatists (New Advent)