I'm not entirely sure what (if anything) Lewis hoped to accomplish with the allegory, since most people I know who read the books as kids didn't "get" the symbolism of it until they were older. As proselytizing goes, it doesn't seem to be very effective. On the other hand, the books may have simply been his own way of coming to terms with that particular religious journey -- and the Christian religious elements are only one (though the strongest) religious tradition that appears in the books, as well.