This post was sparked by a comment on real live preacher's blog, but it's something I've been thinking about for a while. I first started thinking about the topic after reading The Inklings - specifically the background to Mythopoeia.
I think it's very interesting that there are so many similarities between the world's religions, both ancient and modern. Most religions/traditional histories include a massive flood. Eve accepted an apple and Pandora opened a box. Both were created as companions to men (although one might say that all that shows is that mythology was created by men).
C.S. Lewis touches on the topic in The Pilgrim's Regress, where pagans who can't receive God's message by reading (they've been taught that reading is bad) receive dreams from him: their understanding is very confused, but the message begins to get through.
The comment on rlp's blog seemed to suggest that this shows that religion is basically cyclic - humanity has one (maybe more) basic belief system, which is recycled and repackaged over time, but never really changes. My greatest objection to that is that Christianity began before Ancient Greek paganism ended, and that Christianinty is rooted in Judaism.
That's not really proof of anything though, when critically analysed. I suppose your interpretation of anything depends on your starting point - your axioms. For me, God is an axiom. As one of Cynthia Voight's characters said, "Asking me if I believe in God is like asking me if I believe in my own spinal cord." Obviously God exists, Christianity is true and the world falls into place around that.
Maybe the dreams C.S. Lewis wrote about are a way of changing people's axioms. I'm not sure, but it's definitely something to think about. How unchangeable are axioms anyway?