From Mere Christianity

Just read this tonight and it makes sense. I'm taking this from C.S. Lewis' Mere Christianity, Book II: What Christians Believe, Chapter 4: The Perfect Penitent. Keep in mind that Lewis wrote this at a time when it was common to refer to humanity as "Man" and "Mankind." Here he asks "what was the sort of 'hole' that man had gotten himself into?" He answers (Lewis, not Mankind):

"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 - this movement full speed astern - is what Christianity calls repentence. Now repentence 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 to."

Christians believe that Jesus was that perfect person.