maybe religions would have unique units, but those units only fight in wars against other religeons. for instance:
Cristianity - crusaders
Buddhist - warrior monks (apparently there were some.)
Islam - sorry, but Al-Qaida comes to mind
(help me out here)
That's quite racist (not that Islam is a race, or, for that matter, Christians). Which is why religious differentiation will never, and should never, be included in the game. Besides, religions don't actually, by themselves, create units. Having this would undermine why a civ chooses a religion, and just make it about what unit would be helpful at a given time.