OK, let’s start off by not making the mistake enshrined in the title: using the terms ‘fake’ and ‘true’ is going to get somebody’s hackles up eventually, and it might even be somebody who you would feel bad about offending. If you, personally, have a real-person problem with a specific religion, you probably shouldn’t reference it in your game (either as a player or a gamemaster [GM]); for that matter, if you have a problem with religion in general, you probably shouldn’t bring that up, either. I know that that sounds judgemental of me, mostly because it is judgemental of me and I’m personally OK with that. I’m sure that specific people can handle it, but this is a more general advice article.
So… when you have religions in your role-playing game (RPG), should you use real-life religions, or deliberately invented ones? -- Because if you’re playing an RPG about human beings, religion should be in there somewhere. Belief in the supernatural and/or the metaphysical has been a constant concept in human societies, explicitly including the ones that attempted to merely enshrine ‘Reason’ or ‘The Dialectic’ or ‘Juche’ or other subsitutes. And surely I don’t need to note that religious beliefs have influenced countless decisions and movements throughout recorded history, yes? Seriously, in game terms religion is useful for any (GM) interested in complicating his players-characters’ (PCs) lives. No sense in leaving that tool in the campaign toolbox.
So, if you’re now provisionally accepting that religion should show up in a game… there is one major potential problem with using real-life faiths: there’s a non-trivial chance that somebody might get upset. And that’s a problem that can have a remarkably ecumenical scope. Back in the day, one of the go-to guides for incorporating actual Western magical traditions into your RPGs was Isaac Bonewits’ Authentic Thaumaturgy; and when the issue of theurgy (divine magic, like the kind that the AD&D clerics used to use) Bonewits was pretty insistent that it was just as offensive to use real non-Abrahamic religions as it would be to use Abrahamic ones*. Of course, these days the statement should probably be “it is just as offensive to use real Abrahamic religions as it would be to use non-Abrahamic ones,” but that’s a completely different article, and not one that’s necessarily about gaming.
But if using real-life religions is so fraught, then why even entertain the possibility? Blunt answer? Because deliberately made-up religions are, typically, not really as interesting or complex as real ones. Even the ones that come close are not going to have the same attention to detail—and, given the way that devout faiths can shape a person’s daily life, ‘attention to detail’ is important. If you’re trying to get inside the head of your character, and your character is at all religious (a very common condition, throughout history), it really does help if that character believes in something that’s just a touch meatier than Crystal Dragon Jesus.
So, if there are good reasons to avoid real-life religions in-game, and good reasons to use them: how to minimize potential conflict around the gaming table? Actually, I have a specific suggestion. Have everybody, before the start of the campaign—and this includes the GM—sign a written statement saying I promise not to fly off of the handle if I see somebody doing something weird with, or to, a religion that I’m personally fond of. Note that this is not ‘I promise not to disapprove’ or ‘not to say anything;’ it’s merely a promise to not cause a scene, right then and there. Obviously, this isn’t legally binding or anything—but people will still take it more seriously because, hey, they signed the paper. And if someone doesn’t want to sign it then you know that this is a topic that will distress your fellow players (or your GM). That can be helpful information, too.
*If I remember correctly, Bonewits summed up the issue by asking something along the lines of how many hit points St. Peter had. Ironically, Steve Jackson Games (the game company that published Authentic Thaumaturgy) also published In Nomine, which is a game about angels and demons—and thus one where precisely that question might at some point come up in play. Even more ironically, In Nomine’s answer to that particular question was “It’s up to your GM.”