28 But it is possible that you may expect of me that I should go on to refute what he has introduced about Manichæus. Now, in respect of this, the only thing that offends me is that he has censured a most pestilent and pernicious error— I mean the heresy of the Manichæans— in terms of wholly inadequate severity, if indeed they amount to censure at all, though the Catholic Church has broken down his defenses by the strongest evidence of truth. For the inheritance of Christ, established in all nations, is secure against heresies which have been shut out from the inheritance; but, as the Lord says, "How can Satan cast out Satan?" so how can the error of the Donatists have power to overthrow the error of the Manichæans?
Source: Answer to Letters of Petilian, Bishop of Cirta (New Advent)