E.D. Kain tackles the question:
I think fantasy is not really bound to any religion so much as it is bound to a particular way of looking at the world. Somehow the faeries from the old English countryside infected the intellectualism of Oxford’s finest minds. Somehow the old magic of Merlin survived Christianity and became part of the mythical world-building of Tolkien and Lewis and the many non-religious fantasy authors who followed them.
Alyssa Rosenberg complicates this thought:
That we see certain things on the market doesn’t mean that fantasy is limited to those things, or inherently grows out to those things. It just means that we’re reliant on old patterns.
Adam Serwer also chimes in. Kain follows up.
The Dish 