The central Christian belief is that Christ's death has somehow put us right with God and given us a fresh start. Theories as to how it did this are another matter…. The one most people have heard is the one I mentioned before—the one about our being let off because Christ has volunteered to bear a punishment instead of us… And what possible point could there be in punishing an innocent person instead?… On the other hand, if you think of a debt, there is plenty of point in a person who has some assets paying it on behalf of someone who has not. Or if you take ‘paying the penalty’, not in the sense of being punished, but in the more general sense of ‘standing the racket’ or ‘footing the bill’, then, of course, it is a matter of common experience that, when one person has got himself into a hole, the trouble of getting him out usually falls on a kind friend.
Now what was the sort of ‘hole’ man had got himself into? He had tried to set up on his own, to behave as if he belonged to himself. In other words, fallen man is not simply an imperfect creature who needs improvement: he is a rebel who must lay down his arms. Laying down your arms, surrendering, saying you are sorry, realising that you have been on the wrong track and getting ready to start life over again from the ground floor—that is the only way out of our ‘hole’. This process of surrender—this movement full speed astern—is what Christians call repentance. Now repentance is no fun at all. It is something much harder than merely eating humble pie. It means unlearning all the self-conceit and self-will that we have been training ourselves into for thousands of years. It means killing part of yourself, undergoing a kind of death. in fact, it needs a good man to repent. And here comes the catch. Only a bad person needs to repent: only a good person can repent perfectly. The worse you are the more you need it and the less you can do it. The only person who could do it perfectly would be a perfect person—and he would not need it.
Remember, this repentance, this willing submission to humiliation and a kind of death, is not something God demands of you before He will take you back and which He could let you off if He chose: it is simply a description of what going back to Him is like. If you ask God to take you hack without it, you are really asking Him to let you go back without going hack. it cannot happen. Very well, then, we must go through with it. But the same badness which makes us need it, makes us unable to do it. Can we do it if God helps us?
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 54, 56-57.