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.

C. S. Lewis - Mere Christianity, Book II What Christians Believe, Chapter 10 "The Practical Conclusion"