Too many comments to read, so forgive me if I repeat what others have said, but Ursula LeGuin has got to have some good stuff in there, wot? I don't know my LeGuin very well (never read Left Hand b/c I'm a slacker) but Changing Planes comes to mind, although those cultures don't interact with each other.

Also, I'd recommend Ring of Swords by Eleanor Arnason and The Mote in God's Eye by Niven and Pournelle. I believe these both concentrate on the meeting of two cultures, rather than two religions, and I don't remember how big a part religion plays, but the first especially might be worth a look - I can lend, if you want!