In Season 11, Episode 10, when
the Ux apparently decided he was the Creator.
But he was hardly the only person to land on the planet, and the Ux seem to be knowledgeable about Doctor Who "technology" (they recognized the TARDIS as "dimensionally transcendental," for instance). It also seems like they wouldn't assume that anyone who could move from one place in space to another was divine (give that themselves had the same power, to start with). Now, clearly the Ux are, as the Doctor says, "faith-based," but they're not unintelligent.
The one piece of information we have is that:
ANDINIO: The Ux have been waiting for him for generations.
But then, here on Earth many religions have been waiting for their saviors and messiahs for a long time, and they don't accept just anyone who claims to fit the description.
What made the Ux think that he was their god? Was it an effect of the planet? Was there some prophecy he fulfilled?