11 These things have I said, brethren, and somewhat at length: yet because charity was to be more earnestly commended to you, beloved, in this way was it to be commended. For if there be no charity in you, we have said nothing. But if it be in you, we have as it were cast oil upon the flames. And in whom it was not, perchance by words it has been kindled. In one; that which was there has grown; in another, that has begun to be, which was not. To this end therefore have we said these things, that you be not slow to love your enemies.
Does any man rage against you? He rages, you pray; he hates, you pity. It is the fever of his soul that hates you: he will be whole, and will thank you. How do physicians love them that are sick? Is it the sick that they love? If they love them as sick, they wish them to be always sick. To this end love they the sick; not that they should still be sick, but that from being sick they should be made whole. And how much have they very often to suffer from the frenzied! What contumelious language!
Very often they are even struck by them. He attacks the fever, forgives the man. And what shall I say, brethren? Does he love his enemy? Nay, he hates his enemy, the disease; for it is this that he hates, and loves the man by whom he is struck: he hates the fever. For by whom or by what is he struck? By the disease, by the sickness, by the fever. He takes away that which strives against him, that there may remain that from which he shall have thanks. So do thou. If your enemy hate you, and unjustly hate you; know that the lust of the world reigns in him, therefore he hates you.
If you also hate him, you on the other hand render evil for evil. What does it, to render evil for evil? I wept for one sick man who hated you; now bewail I you, if you also hate. But he attacks your property; he takes from you I know not what things which you have on earth: therefore do you hate him, because he puts you to straits on earth. Be not straitened, remove to heaven above; there shall you have your heart where there is wide room, so that you may not be straitened in the hope of life eternal.
Consider what the things are that he takes from you: not even them would he take from you, but by permission of Him who “scourges every son whom He receives.” He, this same enemy of yours, is in a manner the instrument in the hands of God, by which you may be healed. If God knows it to be good for you that he should despoil you, He permits him; if He knows it to be good for you that you should receive blows, He permits him to smite you: by the means of Him He cares for you: wish that he may be made whole.
Source: Homilies on the First Epistle of John (New Advent)