Religion as a mental illness

[robvalue]

I would very roughly hazard a guess that mental illness would be classed as something with a physical cause/imbalance, most likely in the brain. So mental illness can cause delusions.

But people can clearly delude themselves without there being anything physically wrong with their brain. We have ex-theists who had convinced themselves god was speaking to them, or communicating in other ways. Looking back they can see the experiences for what they were, but at the time they seemed real. (I'm not trying to poke fun at them at all.)

So I think this is the line between partial self-delusion and full blown actual delusions. My question is, can someone who doesn't have something physically wrong with their brain develop full blown delusions? Or is it just an advanced level of self-delusion for the theists who really think they see/hear things?