Maybe I am more ‘tolerant” than a lot of people, but If i read a fantasy book written by a Buddhist, (Such as “The Single Shard, a children’s Book), I don’t find it odd at all that the characters would exhibit a Buddhist World-View. If a Fantasy Book (Such as Rick Reardon’s wonderful kid’s books) use the World-View of the Ancient Greek Olympians, and thus educate Children about some of the greatest myths in our culture, I don’t find it odd either.
So, if C.S. Lewis, an unabashed “Mere Christian”, inserts a Christian World view into Narnia, I don’t find that odd at all.
Writers of fantasy bring so much to the table when they create their Alternate Universes. Roger Zelazhny drew heavily upon Ancient Egypt, and George Lucas draws upon the Yin and the Yang.
Everything comes from something. Fantasy writers draw from the incredible wealth of human culture and experience as well as from hard science. Why should a Christian World View somehow seem “odd” to the sci-fi-fantasy genre?
Yes, but I think it fair to say that he didn't write "Creatures of Light and Darkness" with the objective of convincing some of his readers to worship Ra; whereas in the case of Lewis, it seems painfully obvious that he is trying to inculcate Christian values in his young readers. While I certainly don't have a problem with that as such, I think that in this case it sometimes gets in the way of telling a good story.
There are certainly fantasy authors who use explicit Christian theology in their work without any attempts at proselytization. Just as a for instance: "Her Majesty's Wizard", by Christopher Stasheff.
Why should a Christian World View somehow seem “odd” to the sci-fi-fantasy genre?
I think it's safe to say that when it comes to incorporating religious themes into sf/fantasy, most authors treat Christianity as Just Another Religion, albeit one with enormous cultural influence. For a writer to explicitly put Christian themes into his/her work with the objective of converting the reader is extraordinarily rare. I can't think of a mainstream sf/fantasy author beyond Lewis who does so (I wouldn't count Tolkien; his work is far more subtle than Lewis' when it comes to incorporating morality).