At first I had very little idea how the story would go. But then suddenly Aslan came bounding into it. I think I had been having a good many dreams of lions about that time. Apart from that, I don’t know where the Lion came from or why He came. But once He was there He pulled the whole story together, and soon He puled the six other Narnian stories in after him. C. S. Lewis, “It All Began With a Picture…” Of Other Worlds: Essays and Stories (Harvest Books 2002), 42.
The Chronicles of Narnia are immensely popular and immensely influential fiction for kids. And that’s more significant now than it was at the time–given the increasing antipathy to the Christian faith in contemporary Western society. Mind you, it would lay a firmer foundation if kids got their theology from Bunyan (The Pilgrims Progress, The Holy War)–in contrast to the “mere Christianity” that Lewis allegorizes in The Chronicles of Narnia. Nevertheless, The Chronicles of Narnia probably expose many unchurched children to rudimentary Christian theology–planting seeds to bloom later on.
It’s interesting that the stimulus wasn’t just mental images, but dreams. Given the new covenant promise in Acts 2:17-18, perhaps this was a revelatory dream. Maybe God gave Lewis that key character because the resultant novels would have a generally salubrious effect in reaching many for the Gospel, or preparing their minds. God uses vessels of clay to achieve his aims.