Actually, Shatner himself has said initially, he intended the God pretender to be the devil and that would mean God exists by inference. It's pretty well documented in most of the interviews of the time and in his own "making of" book(s).
William Shatner: "With Harve and the studio suits both worrying that my story, featuring appearances by both God and Satan, would more than likely offend a lot of moviegoers, Harve came up with the idea that perhaps we should alter the story and turn God and Satan into an evil alien pretending to be God for his own gain. This was a huge change, lightening the script considerably, and as I look at it now I can clearly see my acceptance of this most basic revision as my first mistake."