Religion as more of a support than a driving force. I like the idea. Sort of like an exalted faction?

You can't really be a part of an opposing faction, but if you're buddies with one then you can at least gain small benifits (on top of the ones you recieve from your main choice of course). Am I in the ballpark?

Yes Kenji you are close. There will be faction wars against other factions but there will be teams or guild like systems that one can join on top of the religion. The game is planned to be a CORPG with lobbies for characters to interact and such. But this is a long way down the road :)

Are the religious factions independent of (and possibly opposed to) the guilds? Or do the religions overlap/contain one or more guilds each? This brings to mind Crusader Kings (and real history) where the game could easily become not a war between kingdoms but a war between religions (the Catholics, "heretical" orthodoxies, and Islam when the Crusades came about) where the religious leaders themselves were effectively in control of each kingdom.

Or is religion just back story or the justification for various classes to have magic powers?

ADDENDUM (after seeing your most recent post): so does each religion have a control structure (a group of leaders) attached to it? Or do you mean to eschew that part?

The guild leaders will have the option to affiliate themselves with religion if they want to. The religious leaders have a HUGE impact on the story. What will that be? We shall see ;) As for the control structure, the countries will have kings and such and religions will have Head priests and such but a religious leader wont lead a nation.