Your natural life is derived from your parents; that does not mean it will stay there if you do nothing about it. You can lose it by neglect, or you can drive it away by committing suicide. You have to feed it and look after it: hut always remember you are not making it, you are only keeping up a life you got from someone else. In the same way a Christian can lose the Christ-life which has been put into him, and he has to make efforts to keep it. But even the best Christian that ever lived is not acting on his own steam—he is only nourishing or protecting a life he could never have acquired by his own efforts. And that has practical consequences. As long as the natural life is in your body, it will do a lot towards repairing that body. Cut it, and up to a point it will heal, as a dead body would not. A live body is not one that never gets hurt, but one that can to some extent repair itself. In the same way a Christian is not a man who never goes wrong, but a man who is enabled to repent and pick himself up and begin over again after each stumble—because the Christ-life is inside him, repairing him all the time, enabling him to repeat (in some degree) the kind of voluntary death which Christ Himself carried out.
Quotes from Mere Christianity, Part 28
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 62-63.