There's three honest-to-$deity religions which are sourced from sci-fi fandoms. Scientology is the most notorious; Gor is also pretty clear-cut, though it's not always referred to as a religion. Then there's
Meanwhile, the Lovecraft fandom has produced the mother of all religious hoaxes in spite of the author being an atheist. (Someone at the Weird Symposium described Lovecraft's world as being a sci-fi speculation based on Darwin and the emergence of atheism. Hm).
Then there's fandom out of religion, a topic which popped up on LJMQ today with winning timing wrt Christian fanfic. The Arthurian mythos is another tangle, though it seems to be moving in the opposite direction - scads and scads of stories first (stories that were steeped in Christianity anyway), and now you meet pagans who incorporate it into their religious beliefs - or insert the myths into a made-up English history that has it that their newborn religion is really thousands of years old. (Many of these people believe Stonehenge and Britain's other ancient monuments were designed by Druids, which is equally untrue; the only truth you'll find at Stonehenge is the stones themselves).
There's a website that claims to expose an Arthurian cultist who operated in New York in the 80s and 90s, one who bases her practises largely on roleplaying games and on, $deity help me, the Dark Is Rising fandom. See here:
The bulk of her ideas, concepts, and practices in regards to metaphysics were obviously derivative of popular fiction; she went so far as to describe Susan Cooper's The Dark Is Rising cycle of books as a loosely fictionalized account of events in her life. She alleged that Ms. Cooper had been a "friend of the family", especially the Druid/Voudon/Archaeologist father (The name 'Merriman' is actually taken directly from these books, as is the name 'Bran') and had fictionalized the exploits of her and their lives accordingly, occasionally swapping the genders of characters (which was her justification for the lead character in the series's being male).
She did not claim that I was a reincarnate of an obscure figure with a somewhat tangential connection to Arthurian mythology (Gwion Bach) until some time later.
She was also the main force behind the structure of the group, which was again heavily influenced by the structure of RPG-style 'adventure parties' (I repeatedly insisted that this be abandoned; she adamantly maintained that there was a much finer line between the RPGs that she/we played, and reality, than most people thought. She also insisted that fictional representations in popular books and movies were more useful as metaphysical education than that of established traditions. During the second half of our association, her insistence that White Wolf's "Kindred Of The East" source-books were based on fact, as well as the books of Tim Powers--going so far as to designate one of our 'magickal operations' around a plot-line lifted directly from them--proved to be a breaking point for numerous people.)
That's all pre-internets, not within the same territory I was talking about yesterday. The most famous internet fancult is probably the Snapes On An Astral Plane folks. They are religiously devoted to a Harry Potter character, who they believe to be literally real and able to contact them in their dreams. The wank report is too perfect to be summarised or excerpted, so I urge you to go read it.
[edit: I meant to add this yesterday but forgot; an intriguing comment from the F_W thread. "To me, getting a tat of a character is the same thing as getting a tat of any symbol-- it serves as a permanent reminder to embody certain characteristics, or to uphold a values system, or whatever. I have the Triforce tattood on my back, and it's there because what it stands for (the joining of Courage, Wisdom, and Power) is powerful and has a lot of meaning for me. But on the other hand, I also don't think the Triforce exists in some mystical realm, or that Link is sending me signals to write crappy fanfic and make godawful manips in his name. THAT'S what's totally batshit here."]
Then there's the Hojo.org Public Warning, a site which describes the misdeeds of two women, Jen and Renee, whose occult practices involve the Final Fantasy VII fandom, and who use the fandom as a way to make converts. Some dispute this exposé - I've seen a few comments around alleging that it's all just grudgewank on the part of the site owner, possibly taking inspiration from the Madison website I quoted above. I am certain that it's at least broadly true, because I briefly knew Jen Sagan myself, in early 2003. She was the 'friend' of...an emo nuisance, really, who got me nattering to her on MSN. [I would love to know if the blue person recalls any of this.] She seemed quite pleasant and reasonable, and I'd count her as one of the two most evil people I've ever met.
The site's mostly about allegations of appalling behaviour towards housemates, but fancultishness is never far away; "jen was on about how she had been betrayed, how they were so united...about this time i began to read a few horror stories. jen had made aeris [another member of the cult, real name Angel] sit in a bathtub full of ice cubes and green food colouring as part of her 'cetra training.' it's a wonder the poor girl didn't get hypothermia."
It also quotes something she wrote about herself on LJ once:
Hi, my name is Jen, and I help rehabilitate vampires and assist in spiritual awakenings. I also take care of metaphysical emergencies and, oh yeah, I do exorcisims and banishings too. I'm quite versed in ancient ritual and I often use my own blood to seal spells. I'm not catholic, but I'm not wiccan. God talks to me and tells me that the end of the world is at hand, she says. I'm married to Metatron and, oh yes, I'm the physical embodiment of the angel Uriel. In past lives I've been Integra van Helsing, Sephiroth, and Dilandau Albatou, amongst others. I've been to many theripists, but they all keep telling me I'm okay. Aside from catholic priests wanting to exorcise my house and my husband, things are pretty normal. Unless you count having 20 some kids live in the mental realm that my husband and I share as NOT normal...
I sing, I do celtic, modern, ritual and sword dancing, and I'm training to be the next soprano sorceress. Famous people I'm related to; Finn MacChumhal (McCool),Morgan LeFaye, and Bram Stoker.
I have no father, and my mother isn't human. My crazy Uncle Michael works for MI-5, and my Best guy friend was created in a super secret laboratory in Glendale, California, and "born" in the same hospital I was in Pasadena, which leads us both to believe that I might be part of the same "project". My husband, by the way, besides being Metatron, is also a No Life King. Not a nosferatu, no, but an echthros. And a mad scientist. And an Emperor. And a Priest.
(The whole otakukin business is helpfully explained in humanese here).
Dunno that there's any point to make here, other than that there's a slippery slope. There are fans who play with love and dreams, who put their all into it, who let it live inside them, and there are fans who get swallowed by it. And there are religions, which are mostly old and of murky origin, but the stories they tell are supposed to have some different value, morally and religiously, than the stories told in fandom. You get Christian proselytisers handing out copies of St Mark's Gospel, telling you to read the story, because then you'll believe. What's with that?
(My own view is that the Things Out There are unlikely to conform to any shape that comfortably fits in my dear little head, so I may as well grok it however's most appropriate for me. The Tao that can be named is not the Tao, and all that).