The issue is more complex. I first encountered this view when reading Against Our Will by Susan Brownmiller, the seminal book on rape. She listed about seven characteristics of a father rapist. Religiosity was one element. There were several similar ones that made no sense to me. I was curious.
I've asked mental health professionals what it means in context. It is in the DSM-IV diagnostic manual as a criteria for diagnosis for certain disorders. Schizophrenia is one such illness. One person with good credentials explained that excessive religious belief, far more than the norm in the culture, is a hallmark of several disorders. It is a class presentation. I don't know whether the religiosity sparks the illness or the reverse. When I asked another person with the same credentials, I was told that it means excessive religious thought that is not the norm for the individual. If someone is always religious, it is all right. If it is sudden, pathology may be present.
The first prof'l was American while the second one was Indian. I suspect that culture plays a role. I wonder what a cultural anthropologist would say.
I still don't understand how religious belief influences mental illness. Perhaps secular mds have a bias. I don't know.
Personally, I believe culture must be an influence. The South has its Bible thumpers more than the North. I don't believe Southerners are more mentally ill.
What do you think?