"Religion is the belief ABOUT the existence of a superhuman power."
(As opposed to IN the existence of a superhuman power.)

No, it's not. By your definition, if I'm a Protestant, and I don't
believe in Thor, by virtue of that belief about Thor, now I'm a beliegver
(believer) in Norse mythology. That's simply silly.

There is a difference between saying, "I believe there is no god,"
and "I do not have within me the belief in the existence of a god." The
latter is not a belief in, or about, any kind of power; it is the total
absence of belief. You cannot turn that into a religion, no matter how
many times you repeat your thesis.

jms