In the essay, Sometimes Fairy Stories Sometimes Says What’s Best to be Said, C.S. Lewis mulls on the reason for the Form that was taken by the Narnia stories.
He opines “[t]hen came the Form. As these images formed themselves into events (i.e., became a story) they seemed to demand no love interest and no close psychology. But the Form which excludes these things is the fairy tale. … On that side (as Author) I wrote fairy tales because the Fairy tales seemed the ideal form for what I had to say” (collected in Essay Collection and Other Short Pieces, Lesley Walmsley ed. ( Harper Collins, London, 2000) p 527 (hereafter Essays)). For Lewis the fairy tale removes the psychological aspect and largely any human aspect.
It is a vehicle, essentially, to deliver a message, one which has fallen out of favour with its original adult audience (though arguably this had happened in the eighteenth and nineteenth centuries in Western Europe). In ‘On Three Ways of Writing for Children’, whilst discussing the need for morality in children’s fiction, he writes ‘[L]et the pictures tell their own moral. For the moral inherent in them will rise from whatever spiritual roots you have succeeded in striking during the whole course of your life” (Essays p 513). Tolkien and Macdonald trust that the reader will come to the moral in their own fashion but Lewis implies that the writer must insert a moral into the book if they have any spirituality (preferably Christianity).
These both, at one level, extend his view of mythopoeia which is far more reductionist that Tolkien. Whilst Tolkien saw Story as vehicle for expressing the mythological, and from that moving into Christianity, his books do allow for a certain humanity. In the essay, ‘Christianity and Literature’ (Essays pp 411- 421), Lewis decries any approach that looks to the beauty in the work of art rather than the moral which it contains. As with his friend, he looks to literature to approach a deeper truth but I think his difference (perhaps evangelism is too strong a word) comes from his faith. Tolkien never lost his own deep faith whereas Lewis had begun to re-find his during the 1930s (hence the Apologetics).
Perhaps this is why, to my mind, the Narnia books are superficially thin (though the Silver Chair has a wonderful Mediaeval depth to be posted about later). They can clearly fit into the current views of the Christian Fundamentalist movement from the creation of Narnia and its inhabitants by Aslan in Magician’s Nephew to the abandonment of the non-believers in Last Battle. Lewis rights from the enthusiasm of refreshed belief and this does have a lasting impact on the writing.
In part, Narnia is perhaps the last gasp of the Victorian Christian tradition, taking MacDonald to its logical extreme. Middle Earth has the challenge of dealing with the twentieth century, straddling the old and the new.