Source:
Warren's analysis of the adherents.com listing of science fiction writers at URL:
http://www.adherents.com/adh_sf.html

affiliation	adherents.com list	Warren's abbreviated list
atheist	11	11
agnostic	4	4
Humanist	3
deist		3
Lapsed Catholic	1
unknown	14	11
Christian	15	10
Jewish	4	2
LDS	6	4
religious	3
Hindu	1
Tenrikyo	1
other		5
Scientologist	1	1
--
(total)	64	51

Warren explained his criteria for dropping 13 writers thus:

"I cut out anyone who doesn't write science fiction - Shatner and Lucas use ghost writers, Tolkien writes fantasy - or who wrote just one minor science fiction book. This hit more conventional religions worse than it hit atheism, but in fairness I did knock out Sagan."

(Despite the loss of Sagan, the tally of atheists remained unchanged at 11, because Arthur C. Clarke was reclassified from "deist or atheist" to "atheist," based on his write-up on the celebrity-atheist page.)