The Emperor of Mankind preached the Imperial Truth: Atheism, Logic, and Humanity. There are no gods. Which is a lie because gods do exist.

Then he was betrayed, mostly killed, and enshrined on the Golden Throne where 10,000 psychics are sacrificed per day to keep him alive.

And then his empire started preaching the Imperial Creed: That he is the God Emperor. A title he tried desperately to avoid.