You give the prefix to those who don't believe instead of those who do.

This means you need to the "idea" of ANYTHING in order to have UNbelief in said thing.

In reality, I'm just existing. You're the one making up shit and are therefore worthy of the prefix.

Or you could say I'm sane, and you're INsane.

Suddenly I'm an UNbeliever because you made up the flying spaghetti monster?

Nah.

See how that works?