(written from a Production point of view)
The God Thing, by Gene Roddenberry, was one of numerous never-produced scripts that were written with the intention of being made as the first Star Trek movie. It served as the precursor of likewise-undeveloped Star Trek: Phase II pilot episode "In Thy Image" and, ultimately, Star Trek: The Motion Picture.
"Generally, the situation is that the five year mission is over and that it has been over for some time. Most of the regular crew have been promoted and, for the most part, are pretty unhappy with shuffling papers and other administrative jobs. Scotty has become an alcoholic, and McCoy has given up treating Human patients to become a veterinarian, loudly proclaiming animals as the only sensible patients he has ever had […] In the story, there is a threat that brings [these people] all back together again."
In the same "log entry", Starlog noted, "Gene said that the main thrust of the story deals with the meaning of God and whether or not God is much more and further beyond merely some entity that visited the Garden of Eden."
In May 1975, Paramount Pictures announced they would be producing a US$5 million Star Trek movie for theatrical release, with principal photography slated to begin in the fall of that year, and for which Gene Roddenberry was contracted on 12 March 1975, with a budget initially set at US$3 million. Roddenberry returned in May to the same office he once occupied during the production of Star Trek: The Original Series to start the writing process. (Star Trek Phase II: The Lost Series, p. 16; Star Trek - Where No One Has Gone Before, p. 62) It was while he was writing the script, that William Shatner, who was purely by coincidence at the studio on unrelated business, chanced upon Roddenberry and was on the occasion given the beat-for-beat expose on the story outline of The God Thing which he had later recalled in his memoirs. (Star Trek Movie Memories, 1995, pp. 46-49)
Though Roddenberry's first story idea for the film concerned the formative years of the TOS main characters and never made it to the submittal stage, the first script he completed and submitted on 30 June 1975 for the movie concerned a different subject altogether, only to be rejected two months later in August by studio head Barry Diller. (The Making of Star Trek: The Motion Picture, p. 23; Star Trek Phase II: The Lost Series, p. 16) Diller incidentally, had little love lost for Roddenberry ever since he first became personally acquainted with him in October the previous year. Diller was appointed that month by Gulf+Western CEO and Chairman of the Board Charles Bluhdorn as Paramount Pictures head, and one of his first missions he was given, was to look into the possibilities of bringing back Star Trek as a live-action production; Bluhdorn had in the mean time become somewhat infatuated with Star Trek due to the breakout success of The Original Series in syndication, and had adopted Star Trek as something of a pet project. (The Keys to the Kingdom, Chapter 5) Not particularly interested in science fiction in general and Star Trek in particular, Diller nevertheless did not want to antagonize his new boss and his new-found infatuation with Star Trek by refusing and dutifully approached Roddenberry for the project. However, still smarting over a film proposal rejection the year previously (The Cattlemen, based on an original story outline called "A Question of Cannibalism"), Roddenberry had somehow become aware of Bluhdorn's interest and, on instigation of his attorney Leonard Maizlish, decided to play studio politics – at which he was notoriously inept – by holding out on Diller for the better part of half a year. Diller played along – for the time being. (The Keys to the Kingdom, Chapters 2, 5; Return to Tomorrow - The Filming of Star Trek: The Motion Picture, pp. 9, 48) Much to his detriment however, Roddenberry would later find out that Diller, by any standard a formidable executive himself, had a long memory and by no means a man with whom to be trifled.
Apparently unaware of the antagonism he had engendered with Diller, the profoundly atheistic Roddenberry saw it differently as he later explained, "What I think bothered Paramount was that I had a little sequence on Vulcan in which the Vulcan masters, the people Spock studied under, were saying: 'We have never really understood your Earth legends of gods. Particularly in that so many of your gods have said, 'You have to bow down on your bellies every seven days and worship me.' This seems to us like they are very insecure gods.'" (Starlog, vol. 1, no. 2, November 1976, p. 13) Despite the negative response from Paramount, Roddenberry himself was proud of having written The God Thing, remarking, "It gives us kind of a fun look at these people's strengths and weaknesses." (Starlog, vol. 2, no. 3, January 1977, p. 60)
It was later announced, in "log entries" in Starlog (vol. 2, no. 3, January 1977, p. 60), that Roddenberry was adapting his first script treatment into novel form for Bantam Books. In the same "log entry", Starlog reported, "Though confident of publication in the near future, Roddenberry wasn't exactly sure when he would complete the book." Ultimately, the novel was never completed.
Aside from some elements surviving long enough in reworked form to ultimately turn up in The Motion Picture, the atheistic theme in particular, Roddenberry himself came to deeply regret his frankness to Shatner at a later point in time, when the latter's film, Star Trek V: The Final Frontier, went into production in 1988. Featuring very similar atheistic themes akin to The God Thing, a livid Roddenberry decried the movie as "apocryphal", readily accepted by the more puritanical and Roddenberry loyalist elements of "Trekdom". (Star Trek FAQ 2.0, chapter 13; Star Trek Movie Memories, 1995, pp. 283-284) Richard Arnold, who was working at Roddenberry's office at the time, was present when the first story outline was delivered to Roddenberry as a FYI (Roddenberry had any and all creative input taken away from him for the films after The Motion Picture, courtesy Diller and his executive staff), later explaining to Shatner why Roddenberry reacted as he did, "So when you came along, though it was years later, with very similar themes, Gene was really hurt. I think it hurt Gene's ego that you finally going to tell the story that he wanted to tell ten [sic.] years earlier. You were about to succeed where he had failed. At the time, Gene's secretary, Susan was making matters worse by walking around the office stating things like "I can't believe it! He stole your idea. Bill's an asshole. Bill's a bastard." So that did not help, and additionally, I know there was a fairly legitimate concern on Gene's part that your sense of humor [remark: in regard to the way the secondary cast was eventually portrayed in the movie] was a little different than had ever been visualized before." (Star Trek Movie Memories, 1995, pp. 289-291)