I believe everyone has their own religion, as Mick said, religion is a belief system, and we all have a belief system no matter if it's a god you praise each morning or a corn flake, if you praise nothing but believe your just lucky to be alive this morning, it's still your religion.
So is religion a form of mental illness? I can't answer that, but, if it is, we're all nuts.

I have a little motto, "I don't suffer from insanity, I enjoy every minute of it"