that's a really good question, but I too, seem to have seen an increase in religious "fanaticism" among those with mental illness. I don't understand the connection.

It's not just with christianity either. Other religions are just as involved, buddaism, muslim, paganism, and more.

Maybe among the paranoia, grandiose dillusion, there is a desire to connect with some sort of supernatural force and people turn to that which they can't explain and can be "brain washed" into believing.

People who are dysfunctional in a mental way, tend to gravitate towards cultish behavior. Religion is a cultish behavior.

Anyway, that's my 2 cents.