The title “God Emperor” comes from the Dune novel series, by Frank Herbert. Somehow it was picked up as a title for Trump, probably from the “the_donald” subreddit or some cheeky people on 4chan/pol.
As far as I know it’s not meant to be sacrilegious or an insult to God Himself, it’s more of a way to really annoy and infuriate the left.
The very clever people on “the_donald” and “4chan/pol” seem to relish in jabbing the left. Since the left naturally seems to be thin-skinned, insecure, and wimpy, it’s a magnet to see who can out-do each other to get under their skin.
That the left can’t seem to “meme” really hurts them too, probably because they don’t have a sense of humor. Even Hillary could never seem to get an insult constructed in a way that would be effective. Trump usually turned it around on her. (”You’d be in jail.”) Even her slogan “I’m with her” was used by Trump when he told his audience that “I’m with you”.
I thought it came from World War II-era Japan.
Moreover, if you ran it together you got “I’m Wither” and that is not a good image to create for ANY tagline or slogan.