24 To take, for instance, this saying of the ancients, "You shall love your neighbor, and hate your enemy," how does Faustus make out that this is peculiar to Moses? Does not the Apostle Paul speak of some men as hateful to God? And, indeed, in connection with this saying, the Lord enjoins on us that we should imitate God. His words are: "That ye may be the children of your Father in heaven, who makes the sun to rise upon the evil and the good, and sends rain on the just and the unjust." In one sense we must hate our enemies, after the example of God, to whom Paul says some men are hateful; while, at the same time, we must also love our enemies after the example of God, who makes the sun to rise on the evil and the good, and sends rain on the just and the unjust.
If we understand this, we shall find that the Lord, in explaining to those who did not rightly understand the saying, You shall hate your enemy, made use of it to show that they should love their enemy, which was a new idea to them. It would take too long to show the consistency of the two things here. But when the Manichæans condemn without exception the precept, You shall hate your enemy, they may easily be met with the question whether their god loves the race of darkness. Or, if we should love our enemies now, because they have a part of good, should we not also hate them as having a part of evil?
So even in this way it would appear that there is no opposition between the saying of ancient times, You shall hate your enemy, and that of the Gospel, Love your enemies. For every wicked man should be hated as far as he is wicked; while he should be loved as a man. The vice which we rightly hate in him is to be condemned, that by its removal the human nature which we rightly love in him may be amended. This is precisely the principle we maintain, that we should hate our enemy for what is evil in him, that is, for his wickedness; while we also love our enemy for that which is good in him, that is, for his nature as a social and rational being.
The difference between us and the Manichæans is, that we prove the man to be wicked, not by nature, either his own or any other, but by his own will; whereas they think that a man is evil on account of the nature of the race of darkness, which, according to them, was an object of dread to God when he existed entire, and by which also he was partly conquered, so that he cannot be entirely set free. The intention of the Lord, then, is to correct those who, from knowing without understanding what was said by them of old time, You shall hate your enemy, hated their fellow-men instead of only hating their wickedness; and for this purpose He says, Love your enemies.
Instead of destroying what is written about hatred of enemies in the law, of which He said, "I have come not to destroy the law, but to fulfill it," He would have us learn, from the duty of loving our enemies, how it is possible in the case of one and the same person, both to hate him for his sin, and to love him for his nature. It is too much to expect our perverse opponents to understand this. But we can silence them, by showing that by their irrational objection they condemn their own god, of whom they cannot say that he loves the race of darkness; so that in enjoining on every one to love his enemy, they cannot quote his example.
There would appear to be more love of their enemy in the race of darkness than in the god of the Manichæans. The story is, that the race of darkness coveted the domain of light bordering on their territory, and, from a desire to possess it, formed the plan of invading it. Nor is there any sin in desiring true goodness and blessedness. For the Lord says, "The kingdom of heaven suffers violence, and the violent take it by force." This fabulous race of darkness, then, wished to take by force the good they desired, for its beautiful and attractive appearance.
But God, instead of returning the love of those who wished to possess Him, hated it so as to endeavor to annihilate them. If, therefore, the evil love the good in the desire to possess it, while the good hate the evil in fear of being defiled, I ask the Manichæans, which of these obeys the precept of the Lord, "Love your enemies"? If you insist on making these precepts opposed to one another, it will follow that your god obeyed what is written in the law of Moses, "You shall hate your enemy"; while the race of darkness obeyed what is written in the Gospel, "Love your enemies."
However, you have never succeeded in explaining the difference between the flies that fly in the day-time and the moths that fly at night; for both, according to you, belong to the race of darkness. How is it that one kind love the light, contrary to their nature; while the other kind avoid it, and prefer the darkness from which they sprung? Strange, that filthy sewers should breed a cleaner sort than dark closets!
Source: Reply to Faustus the Manichaean (New Advent)