Chuck being god was a big fan theory around season 5, there were hints at it, but I think the main one was in the season 5 finale when he's writing the story and doing the narration over the episode and then at the end he just disappears into nothing, if you guys have time I would recommend just watching like the last couple minutes of that episode, also when he comes back during the 200th episode, the one with the play, considering for kevin to be a prophet the previous prophet needs to be dead, which was a whole thing during season 7 and 8.