Today I'm asking all of you who write genre fiction, design fictional worlds for gaming, and so on to head on over to the blog of Dragon Award winner Brian Niemeier, author of Nethereal and Souldancer, because he's got a great post on the matter of believable portrayals of religion in such contexts. This post links to a guest post he did at Ragnarok Publications on the topic, which expands upon the excerpt in the post.
I'm pointing out this post because of what he points out in Kairos post: that attention to detail makes the story (setting, etc.) easier for a reader/player to accept despite it being unreal, and well-done creation allows for exploration of ideas and themes in a manner that is fun for all concerned. Fun, entertaining, and not "(unsolicited opinions on Israel)".
You can have religion be fun in your fictional work, writing and gaming alike, but like anything else you have to put in some work to set it up and make it so. Don't shy away from that; the payoff is spectacular, as we can clearly see in Tolkien's Middle Earth (which is informed by Tolkien's Catholicism without being so) and Lewis's Narnia (informed by his Protestantism). Give it a go, and if you don't feel like making one up wholly then take some existing one and see if you can make it better (e.g. all of any D&D world's religions).