There's a major difference between a deity that people sincerely worshipped and a deity that one guy just made up and nobody ever actually believed in.

Even if you're a hard-core atheist who places Yahweh on the same level as the boogeyman (which describes me pretty well), you should still recognize the cultural significance.

So the only difference is popularity. Religions like Scientology and Mormonism pretty much proved that certain people will believe anything, there's no limit to gullibility.

The popularity difference between millions or billions of followers and zero is important.

Where do you think the "authentic" deity came from? The culture has to be built atop something. It all starts from somewhere.

I'm well aware that this stuff is ultimately all made up. That doesn't change the fact that there's a massive difference between one that's made up and nobody ever took seriously, and one that is or was seriously worshipped by people who sincerely believe it's real.

You only need 12 disciples to start a religion ;)

That's just what they want you to think!

Generally not by encyclopedias though.

Ah, but how would you know?

Yeah, it's totally feasible that some adherent of this religion could've looked up its gods in Wikipedia and now worships this guy for real. Sure, in such case it's a "self-fulfilling prophecy", but it wouldn't be much different from John Frum cult or whatever.

Precisely. All this deity needs is some proselytizing and it'll be no more or less legitimate than any of the other made-up religious characters.