Since C.S. Lewis considers faith to be the most difficult subject he has tackled thus far in Mere Christianity, he gives his readers some encouragement: "There are certain things in Christianity that can be understood from the outside, before you become a Christian. But there are a great many things that cannot be understood until after you have gone a certain distance along the Christian road.... There are directions for dealing with particular crossroads and obstacles on the journey and they do not make sense until a man has reached those places. Whenever you find a statement in Christian writings that you can make nothing of, do not worry. Leave it alone. There will come a day, perhaps years later, when you suddenly see what it meant."
In the last chapter, Lewis talked about faith in the sense of a steady belief in facts. In this chapter, he says, "I am trying to talk about faith in the second sense ... after a man ... discovers his bankruptcy.... It is the change from being confident about our own efforts to the state in which we despair of doing anything for ourselves and leave it to God.... He ... trusts that Christ will somehow share with him the perfect human obedience that He carried out from His birth to His crucifixion.... In Christian language, He will share His 'sonship' with us.... Christ offers something for nothing: He even offers everything for nothing."
"Handing everything over to Christ does not, of course, mean that you stop trying. To trust Him means ... trying to do all that He says. There would be no sense in saying you trusted a person if you would not take his advice. Thus if you have really handed yourself over to Him, it must follow that you are trying to obey Him. But trying in a new way, a less worried way. Not doing these things in order to be saved, but because He has begun to save you already. Not hoping to get to Heaven as a reward for your actions, but inevitably wanting to act in a certain way because a first faint gleam of Heaven is already inside you.... The Bible really seems to clinch the matter when it puts the two things [good works and faith] together into one amazing sentence [Philippians 2:12-13]. The first half is, 'Work out your own salvation with fear and trembling'—which looks as if everything depended on us and our good actions: but the second half goes on, 'For it is God who worketh in you'—which looks as if God did everything and we did nothing."
"Christianity seems at the first to be all about morality, all about duties and rules and guilt and virtue, yet it leads you on, out of all that, into something beyond. One has a glimpse of a country where they do not talk of those things, except perhaps as a joke. Everyone there is filled full with what we should call goodness as a mirror is filled with light. But they do not call it goodness. They do not call it anything. They are not thinking of it. They are too busy looking at the source from which it comes. But this is near the stage when the road passes over the rim of our world. No one's eyes can see very far beyond that: lots of people's eyes can see further than mine."
Highlights from chapter 12: Faith, book 3: Christian Behavior in Mere Christianity by C.S. Lewis. Click here for a clear view of how this chapter relates to the whole book.