Having chatted with atheists I notice many of them seem to have a sense of pride and self-worth to be able to handle life on their own, to eschew the crutch of religion and to stand up and take life's hits all on their own. God, heaven and all of that are simply imaginative playthings for those poor deluded souls who need something to lean on in order to make it through.
Lewis, of course, believed and preached such self-faith resolutely during his atheistic days, so when Christianity finally dragged him into the fold of the faithful, it was natural that he wished to answer the problem.
It's in The Silver Chair that he does so most succinctly and beautifully, if you can see through the veneer of a children's story, that is.
In the story, the Witch holds the prince, two children and a marsh-wiggle captive in her underground kingdom. As part of her attempt to enchant their minds and ensnare the prince whose land above (Narnia) she hopes to overthrow, she uses a sweet, enchanting green powder in her fire, a soothing, sleepy music and a long speech in which she tells the captives how the "real world" exists only there in her underground lair. They made up the sun, describing it as a lamp, the sky is a mere figment and even Aslan, the Great Lion (a Christ-figure in the books) is nothing more than a cat with extra features.
Puddleglum, the marsh-wiggle, responds with what is almost certainly one of Lewis's best answers to the assertion that the elements of orthodox Christianity are merely made-up inventions and empty imaginative traditions and story-myths.
"'One word, Ma'am,' he said... 'One word. All you've been saying is quite right, I shouldn't wonder. I'm a chap who always liked to know the worst and then put the best face I can on it. So I won't deny any of what you said. But there's one thing more to be said, even so. Suppose we have only dreamed, or made up, all those things--trees and grass and sun and moon and stars and Aslan himself. Suppose we have. Then all I can say is that, in that case, the made-up things seem 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 playing a game can make a play-world which licks your real world hollow. That's why I'm going to stand 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 like a Narnian as I can even if there isn't any Narnia. So, thanking you kindly for our supper, if these two gentlemen and the young lady are ready, we're leaving your court at once and setting out in the dark to spend our lives looking for Overland. Not that our lives will be very long, I should think; but that's small loss if the world's as dull a place as you say."
Lewis himself, as outlined in his biography, Surprised by Joy, actually came to Christianity through the back-door of myth and fairy tale. Rather than detracting from religion, Lewis found that the ancient myths, the Norse hero-stories and traditional fantasies were so beautiful, so glorious that they actually opened his eyes to the idea that something bigger, grander, more splendid existed than the mere mundane humanity with which he was surrounded. As he searched through spiritualism, the occult, eastern religions and mysticism, he found that the only thing with any substance to match the awe that existed in the old myths was Christ Himself.
Psalm 19 (New International Version)
1 The heavens declare the glory of God;
the skies proclaim the work of his hands.
2 Day after day they pour forth speech;
night after night they display knowledge.
3 There is no speech or language
where their voice is not heard.
4 Their voice goes out into all the earth,
their words to the ends of the world.
In the heavens he has pitched a tent for the sun,
5 which is like a bridegroom coming forth from his pavilion,
like a champion rejoicing to run his course.
6 It rises at one end of the heavens
and makes its circuit to the other;
nothing is hidden from its heat.