14 And some indeed, who are used to excuse their own sins, complain that they are driven to sin by fate, as though the stars had decreed this, and heaven had first sinned by decreeing such, in order that man should after sin by committing such, and thus had rather impute their sin to fortune: who think that all things are driven to and fro by chance accidents, and yet contend that this their wisdom and assertion is not of chance rashness, but of ascertained reason. What madness then is it, to lay to reason their discussions, and to make their actions subject to accidents!
Others refer to the devil the whole of what they do ill: and will not have even a share with him, whereas they may suspect whether he by hidden suggestions has persuaded them to evil, and on the other hand cannot doubt that they have consented to those suggestions, from whatever source they have come. There are also they who extend their defense of self unto an accusation of God, wretched by the divine judgment, but blasphemers by their own madness. For against Him they bring in from a contrary principle a substance of evil rebelling, which He could not have resisted, had He not blended with that same that was rebelling a portion of His own Substance and Nature, for it to contaminate and corrupt; and they say that they then sin when the nature of evil prevails over the nature of God.
This is that most unclean madness of the Manichæans, whose devilish devices the undoubted truth most easily overthrows; which confesses that the nature of God is incapable of contamination and corruption. But what wicked contamination and corruption do they not deserve to have believed of them, by whom God, Who is good in the very highest degree, and in a way that admits not of comparison, is believed to be capable of contamination and corruption?
Source: On Continence (New Advent)