Tor.com’s Ruthanna Emrys has compiled a list of five recommended works of science fiction concerning invented religions:
Delve deep enough into linguistics, and eventually you’ll want to try constructed languages, with new vocabularies and grammars that illustrate the principles and limitations of those that occur naturally. Spend enough late nights arguing theology, and you start wanting to make up your own. My first-ever business card was for the half-joking Discount Deities: custom pantheon creation and appropriately biased origin myths.
A good fictional religion, like a real one, provides appealing insight into our place in the universe. It may even answer yearnings that existing faiths have failed at—at least for the author, and sometimes for the readers. So, just as many constructed languages accrete communities of speakers, imagined religions spill over into real practice. These new faiths start out serving the purpose of their stories, but not all have stayed fictional.