I came in to post this this morning, but got sidetracked by the intrusive work.
Personally, I would probably opt for a church that promoted sexuality over one that charges admission and hooks you up to a lie detector…if I were shopping around for one…
Science fiction be damned.
There is only One True God.
Gaze ye upon His Yellowness and fear ye His wrath.
Cthulhu fhtagn!
In addition to CAW, there’s the Avalon bunch, who are basically recreating the fantasy recreation of MZB in Mists of Avalon. Actually, I’d estimate about 90% of neo-paganism’s ideas of Goddess religions of now and of olde come from Marion. (Ironic, since she repented her evil ways and became a born-again Christian before her death, I hear.)
If I had to pick a SF writer to form a religion, it would be Harry Harrison.
Because many of his stories are anti-religious.
In The Streets of Ashketon and Rescue Mission, holy men end up being the bad guys. Only they’re not evil, they truly want to do what’s right, only they’re too ignorant to know what that is.
In The Stainless Steel Rat Goes to Hell, Christianity is revived far in the future and the two main characters think it’s silly.
A religion created by a man who is against religion. I like that.
What was the name of the religion in The Sirens of Titan? That’s my favorite.
We’ve gone this far without mentioning Walter Miller, Jr.? Take an ordinary, innoculous artifact out of context and build a religion around it.
While not a science fiction writer, I think Joseph Heller, author of Catch-22 and God Knows, would make an ideal “imagineer” for a new religion, loading it with a sense of the absurd on top of plenty of irong, and iced over with a saccharine frosting of sincere illusion and misdirection.
Or, if you’re into the indecipherable and dystopic, Franz Kafka.
Then again, I’ve always wanted Cthulhu for President (“Why vote for the lesser evil?”), so maybe HP Lovecraft would be a good choice.
Stranger
The Church of God the Utterly Indifferent.
Do you mean Cat’s Cradle? The religion is Bokononism.
From The First Book of Bokonon:
I love Vonnegut.
*“Foma” is defined in the book variously as “lies,” “harmless untruths” and “a useful and harmless sort of horseshit.”
Nope, I meant The Sirens of Titan. kelly5078 nailed it, the Church of God the Utterly Indifferent.
My bad. I forgot about the CoGtUI. 
No one’s up for Philip K Dick religion?
Mercerism, perhaps
According to legend, Mercer had the power to revive dead animals but angered local officials, who attacked him with radioactive cobalt. This forced Mercer into a tomb world. He continues to try to ascend back to Earth, forever climbing an enormous hill. Adherents of Mercerism use an empathy box, an electronic device which instills in its user a hallucination of sharing Mercer’s sufferings. Mercerism blends the concept of a life-death-rebirth deity with the values of unity and empathy, all of which are inspiring to those left on a dying Earth.
Harlan Ellison’s version of the Genesis story and subsequent characterization of the subtil serpent in “The Deathbird” is actually pretty close to my preferred allegorical interpretation of the creation myth.
When I was a teenager, my younger brother bought The Necronomicon and really thought it was a religious/mystical text. He wanted to do one of the rituals in it but the necessary components were too weird for him to acquire. For all I know he still thinks it’s real, since I don’t think he’s ever read Lovecraft.
I’m kind of partial the the religion of Elua in Jacqueline Carey’s Kushiel books, myself.
There are already people who call themselves Jedi …
That’s got to count as an SF religion, surely?
I think I could accept The Last Question by Asimov as a religious text.
I gotta go with HH as well, for his conceptualization of Individual Mutualism (The Stainless Steel Rat Gets Drafted). Not really spiritual, but a darned good way to live.
Especially if you couple it with Heinlein’s Rational Anarchy (The Moon Is A Harsh Mistress).
The voices in my head told me to move this to Cafe Society.
I think The Church of All Worlds would be okay. They know how to party.
Say, what was the name of Niven’s religion with the Dread God Finagle?