[One of the most unpopular of the Christian virtues] is laid down in the Christian rule, 'Thou shalt love they neighbor as thyself.' Because in Christian morals 'thy neighbour' includes 'thy enemy', and so we come up against this terrible duty of forgiving our enemies. Every one says forgiveness is a lovely idea, until they have something to forgive, as we had during the war. And then, to mention the subject at all is to be greeted with howls of anger. It is not that people think this too high and difficult a virtue: it is that they think it hateful and contemptible. 'That sort of talk makes them sick,' they say. And half of you already want to ask me, 'I wonder how you'd feel about forgiving the Gestapo if you were a Pole or a Jew?' So do I. I wonder very much. Just as when Christianity tells me that I must not deny my religion even to save myself from death by torture, I wonder very much what I should do when it came to the point. I am not trying to tell you in this book what I could do--I can do precious little--I am telling you what Christianity is. I did not invent it. And there, right in the middle of it, I find 'Forgive us our sins as we forgive those that sin against us.' There is no slightest suggestion that we are offered forgiveness on any other terms.
C.S. Lewis, Mere Christianity (1952)