It is widely believed that L. Ron Hubbard and Robert A. Heinlein made a bet in a bar one night either that L. Ron could not create a religion, or to see who could create a religion first. (In the second case, Stranger in a Strange Land is often cited as Heinlein's effort.)
Ignoring calling any religion's validity into question; is there any evidence for or against this bet actually happening? (Perhaps L. Ron made the bet because it was a sure thing, betters can also be prophets.)
UPDATE:
This article seems to be the most comprehensive. The summary:
The simple truth of the matter is that a wager never took place. It never happened, end of story.
Having said that however, the possibility does exist that a series of conversations between the two authors and commentary made by Heinlein, may have been the catalyst for Hubbard’s authorship of Dianetics. Heinlein and Hubbard were close friends and Hubbard greatly respected Heinlein, his opinions and his ideas.
Mr. Patterson revealed to me, "RAH and LRH had one or more discussions during 1944 and or 1945 when they were both in Philadelphia, and RAH pointed out to LRH that religions had an inordinate amount of legal latitude in the U.S. and that churches could engage in a great many activities otherwise thought of as secular, under the tax and other protection churches enjoy. He had already explored these ideas in some of his stories and was to revisit these notions in their original form in Stranger. It is possible that this conversation or series of conversations took place as late as December 1945 or early 1946 and in Los Angeles."
As per Wikipedia article on bar bets (... from the "last places you'd think to find things" category :)
It is widely believed that the creation of Scientology was the result of a bar bet between L. Ron Hubbard and Robert A. Heinlein. The story says L. Ron Hubbard dared that he could create a religion all by himself. According to Scientology critic Lindsay this is "definitely not true", no such bet was ever made, it would have been "uncharacteristic of Heinlein" to make such a bet, and "there's no supporting evidence". However, several of Heinlein's autobiographical pieces, as well as biographical pieces written by his wife, claim repeatedly that the bet did indeed occur.
Please note that the "biographical pieces" does NOT refer to the main RAH "biographical" book, "Grumbles from the Grave" - a full text search found no mentions of 'bar', 'bet', 'religion', 'hubbard' or 'dianetics' in the desired context.
The only other place it can be is a bunch of letters in RAH/VH archives, but those cost a lot of money to access online ($3-$12 per archive item) and as such I'm afraid I won't be able to confirm/deny using those; but I included the URL for those who may want to.
Harlan Ellison has stated that he was present when Hubbard made the above statement.
On Wikiquote:
Scientology is bullshit! Man, I was there the night L. Ron Hubbard invented it, for Christ's sakes! ... We were sitting around one night... who else was there? Alfred Bester, and Cyril Kornbluth, and Lester del Rey, and Ron Hubbard, who was making a penny a word, and had been for years. And he said "This bullshit's got to stop!" He says, "I gotta get money." He says, "I want to get rich". And somebody said, "why don't you invent a new religion? They're always big." We were clowning! You know, "Become Elmer Gantry! You'll make a fortune!" He says, "I'm going to do it."
"The Real Harlan Ellison" in Wings (November-December 1978) p. 32
The Harlan Ellison FAQtotum has a larger extract of this interview.
This web page - which I found by searching for the names of two of the witnesses quoted by Russell Miller in his fascinating biography of Hubbard, Bare-Faced Messiah - seems pretty authoritative, and it gives plenty of references:
non-scientologist faq on "Start a Religion"
L. Ron Hubbard is widely rumored to have said "The way to make a million dollars is to start a religion." There are also variant rumors. For some reason, this is often mentioned on Usenet. Evidence is discussed below, but the short answer is that it's almost certainly true.
[...]
One form of the rumor is that L. Ron Hubbard made a bar bet with Robert A. Heinlein. This is definitely not true. It's uncharacteristic of Heinlein, and there's no supporting evidence. There is, however, inconclusive evidence that Robert Heinlein suggested some parts of the original Dianetics.
[...]
To summarize: we have nine witnesses: Neison Himmel, Sam Merwin, Sam Moskowitz, Theodore Sturgeon, Lloyd Arthur Eshbach, Harlan Ellison, and the three unnamed witnesses of Robert Vaughn Young. There is some confusion and doubt about one of them (Sam Moskowitz). Two are reported via Russel Miller: one is reported via Mike Jittlov: one reported in his autobiography; one reported in an affidavit; and one reported to me in person. The reports describe different events, meaning that Hubbard said it perhaps six times, in six different venues - definitely not just once. And the Church's official disclaimer is now reportedly a flat lie.
Conclusion: He definitely said it more than once.
I'd heard this somewhat differently back in the 1970s when I was first getting into Asimov.
The story I'd heard took place in the 50s (but could have been the 60s) allegedly during a dinner party or gathering of several SF authors, Asimov included. At some point, Hubbard was alleged to have said "Religion is where the REAL money is at", or words to that effect.
