The World of Arth
Any cleric can pick any two domains. These domains represent the ideals of their chosen deity, belief system, or passions.
There are tons of minor cults and religious practices around the world, none of them uniform or widespread. Many regions have an official “state” religion, but many make little to no effort on enforcing a specific belief.
Many of these cults claim ties to religions and deities of old as form of legitimacy, but much of the religious texts have been lost with the passing of time and most of the doctrine is based off of incomplete texts.
For players, this means your PC can have no deity, a deity they made up on the spot, one from Eberron, Forgotten Realms, or even one from Greek/Norse mythology.