I imagine somebody will say, ‘Well, if one is allowed to condemn the enemy’s acts, and punish him, and kill him, what difference is left between Christian morality and the ordinary view?’ All the difference in the world. Remember, we Christians think man lives for ever. Therefore, what really matters is those little marks or twists on the central, inside part of the soul which are going to turn it, in the long run, into heavenly or a hellish creature. We may kill if necessary, but we must not hate and enjoy hating. We may punish if necessary, but we must not enjoy it. In other words, something inside us, the feeling of resentment, the feeling that wants to get one’s own back, must be simply killed. I do not mean that anyone can decide this moment that he will never feel it any more. That is not how things happen. I mean that every time it bobs its head up, day after day, year after year, all our lives long, we must hit it on the head. It is hard work, but the attempt is not impossible. Even while we kill and punish we must try to feel about the enemy as we feel about ourselves—to wish that he were not bad, to hope that he may, in this world or another, be cured: in fact, to wish his good. That is what is meant in the Bible by loving him: wishing his good, not feeling fond of him nor saying he is nice when he is not.
C.S. Lewis, Mere Christianity (1952; Harper Collins 2001) 119-120.