The Crusades of the Middle Ages can be very inspiring for ideas in the game. I recently watched the film, "The Kingdom of Heaven", directed by Ridley Scott. The film reminded me that there are a vast range of cultural and religious nuances that can serve as the root of conflict in a campaign, some of which can be very complex, as well as having the apparent superficial quality of seeming to be quite trivial. However, upon closer inspection, even seemingly trivial points of difference--such as obscure theological doctrinal differences--can, when they are practiced and held by whatever culture or people, build up and generate some significant friction, and reason for conflict.
Then, in the game-world where there are often many different religions, the scope for religious conflict expands greatly, not to mention the potential conflicts even within various racial religions, i.e elves, which might develop different philosophical interpretations on the *most important* aspect of a particular deity, or religious precept.
Then again, in my campaigns, while there is divine involvement, and periodically communication to various degrees, it isn't like priests sit down to have coffee with their patron god or goddess. The gods may have said "X" at some time in the past, or inscribed some crazy magical tablet, but it remains for the priests and priestesses to *interpret* such things. That's where I have a lot of fun creating different theologies, and philosophies that drive and inspire different religions, from where they can take a philosophical doctrine, and practice it one way--while using the same philosophical doctrine, a different branch can practice it another way. Both at the same time, remaining seemingly "true" to whatever divinely-inspired doctrine or commandment.
I will expand this article tomorrow.