14 Instead of one good and one evil principle, you seem to make both good or both evil, or rather two good and two evil; for they are good in themselves, and evil to one another. We may see afterwards which is the better or the worse; but meanwhile we may think of them as both good in themselves. Thus God reigned in one region, while Hyle reigned in the other. There was health in both kingdoms, and rich produce in both; both had a numerous progeny, and both tasted the sweetness of pleasures suitable to their respective natures.
But the race of darkness, say the Manichæans, excepting the part which was evil to the light which it bordered on, was also evil to itself. As, however, I have already pointed out many good things in it, if you can point out its evils, there will still be two good kingdoms, though the one where there are no evils will be the better of the two. What, then, do you call its evils? They plundered, and killed, and devoured one another, according to Faustus. But if they did nothing else than this, how could such numerous hosts be born and grow up to maturity?
They must have enjoyed peace and tranquillity too. But, allowing the kingdom where there is no discord to be the better of the two, still they should both be called good, rather than one good and the other bad. Thus the better kingdom will be that where they killed neither themselves nor one another; and the worse, or less good, where, though they fought with one another, each separate animal preserved its own nature in health and safety. But we cannot make much difference between your god and the prince of darkness, whom no one opposed, whose reign was acknowledged by all, and whose proposals were unanimously agreed to.
All this implies great peace and harmony. Those kingdoms are happy where all agree heartily in obedience to the king. Moreover, the rule of this prince extended not only to his own species, or to bipeds whom you make the parents of mankind, but to all kinds of animals, who waited in his presence, obeying his commands, and believing his declarations. Do you think people are so stupid as not to recognize the attributes of deity in your description of this prince, or to think it possible that you can have another?
If the authority of this prince rested on his resources, he must have been very powerful; if on his fame, he must have been renowned; if on love, the regard must have been universal; if on fear, he must have kept the strictest order. If some evils, then, were mixed with so many good things, who that knows the meaning of words would call this the nature of evil? Besides, if you call this the nature of evil, because it was not only evil to the other nature, but was also evil in itself, was there no evil, think you, in the dire necessity to which your god was subjected before the mixture with the opposite nature, so that he was compelled to fight with it, and to send his own members to be swallowed up so mercilessly as to be beyond the hope of complete recovery?
This was a great evil in that nature before its mixture with the only thing you allow to be evil. Your god must either have had it in his power not to be injured and sullied by the race of darkness, in which case his own folly must have brought him into trouble; or if his substance was liable to corruption, the object of your worship is not the incorruptible God of whom the apostle speaks. Does not, then this liability to corruption, even apart from the actual experience, seem to you to be an evil in your god?
Source: Reply to Faustus the Manichaean (New Advent)