But are we talking about adding real-world religions in to MMOs, or creating new religions specifically for games? As examples of the latter; most Priests and Paladins in World of Warcraft follow the Holy Light. The Holy Light isn't actually a theistic religion, in that its followers don't worship a god or gods, instead they look inwards. Its followers try to better themselves, and better the world they live in. The Forsaken, on the other hand, take a rather different view.
Following their emancipation from the Lich King's control, those among the Forsaken who had followed the Holy Light turned away from it and chose another path. They chose Forgotten Shadow, a form of 'divine humanism' where they seek power for themselves, to force change upon the universe as they see fit.
Imaginary religions are one thing. On one hand, they're essentially harmless; I struggle to imagine a religion whose core tenets were essentially racist making it into a big-budget MMO, for example. On the other hand, it's a bit difficult to generate a desire to march to a virtual church and pray to a fake deity or deities – religions created purely for a game will lack 'bite'.
Imagine if you will, a culture in-game which has developed as a Theocracy. For our purposes, it doesn't matter if the culture is despotic or utopian. What matters is that the culture is one where religion not only informs the people's moral and ethical view of the world, but the tenets of their religion defines their daily routine. This kind of thing is common in the real world; plenty of religions have strict rules for their members to follow. But imagine a faction in an MMO where the NPC vendors were unable to sell or buy alcohol? Imagine you'd spent an afternoon levelling up your Cooking skills, only to fin you couldn't sell the products of your labour due to you not having slaughtered the mobs correctly. I'm talking about an in-game religion actively changing the way we play the game.