Suppose we have only dreamed, or made up, all those things - trees and grass and sun and moon and stars ans Aslan himself. Suppose we have. Then all I can say is that, in that case, the made up things seems a good deal more important than the real ones. Suppose this black pit of a kingdom of yours is the only world. Well it strikes me as a pretty poor one. And that's a funny thing, when you come to think of it. We're just babies making up a game, if you're right. But four babies making up a game can make a play world which licks your real world hollow. That's why I am going to stick by the play-world. I'm on Aslan's side even if there isn't any Aslan to lead it. I'm going to live as much like a Narnian as I can even if there isn't any Narnia.
This is easily the most theological statement in a book that is remarkably free of theology (at least in the first 220 pages that I have read so far). This same character makes many statements claiming that nothing happens by accident - a pretty bold claim but less so in a fantasy setting.
From what I have read if Lewis' conversion to Theism the above quote seems almost auto-biographical. Lewis came to the conclusion that if you have a desire for something it must be real. Here he is saying that if what you want to believe sounds better than reality than it is worth behaving as if it were. There is a subtle difference - in the first case he was arguing for the actual existence of something while in the second he was arguing for governing your actions as if something existed. It reminds me of Pascal's Wager, "What do you have to lose if God turns out not to exist?" In Lewis example you should act as if Christ were real not just because it is an appealing thought but also because it makes you a better person.
I find this kind of interesting because I have long believed that one must choose to live as if Theism is true or as if God does not exist. Agnostics can talk about doubt at their dinner parties but they can not live doubt, they have to make a choice.