One of the things that I have always liked about the older version of the world of Greyhawk is that there was always a pronounced bit of disregard when it came to religion in the game world. You could openly worship H.P. Lovecraft's Elder Gods in Greyhawk City and so long as you kept your sacrifices to animals you purchased and to people who were down for getting offered up to the Elders no one gave two shits about what you were doing. Unfortunately that all changed in the game world as TSR moved away from that disregard of religion and instead made the decision that the choice of your religion could color your character in a more pronounced way. As a result you couldn't be an open worshiper of of an "evil" god, such as Nerull, or else some self-righteous paladin from one of the "good" religions was going to make it their life's goal to plant your character's ass into the ground and see if you could grow a better person from your bones.
This decision to make religions matter in this fashion drove a lot of choices that players could have made into these underground and secretive places where a division within the players was built not on who got the most loot but on who was worshiping the wrong, imaginary deity. It infected every aspect of the game as cults always worshiped evil gods bent on doing harm to the whole world and a worshiper of Nerull or Vecna was always up to no good. In a lot of ways it simplified the world into this trite, little thing that mattered less.
Don't get me wrong: there should absolutely be unifying forces in the world, and religion is a powerful unifier; but there is no reason to make the choice of a religion dictate whether a character is a good or evil actor in the game. We have lot of other reasons to kill the latest non-player character (NPC) we come across without dropping an imaginary religion into things as a moral panacea for our actions.
More later.
