Apologies for my brief post which probably didn't really say what I wanted to say. My point was that marygarvey was focussing on the negative associations between mental health and religion, but that I believe the picture is much more complex. I don't have time to pull out all the references, but from what I have read, yes it is true that religious people who suffer from psychosis tend to have stronger religious delusions than non religious people. However that may just be a change in content due to the focus of the person's life. I don't know of any evidence that religious content delusions lead to more serious consequences (eg more violent behaviour) than delusions with other content. Other studies show that many people suffering psychosis use their religion as a coping mechanism, and become more religious after their psychosis appears. These people were more likely to be compliant with anti-psychotic medication.
In the non-psychotic arena, the evidence seems to be that those with religious beliefs have lower levels of depression and suicide. Though of course the direction of this correlation isn't fully understood. Maybe religious beliefs have a positive impact on mental health, maybe people with better mental health are drawn to religion, maybe both religious belief and good mental health are influenced by a third unknown factor. Maybe the relationship is mediated by another factor - eg people with religious beliefs have a larger social network which is the real protective factor.

Bagpuss