Also, most mentally ill people are also deeply religious.
Since there has never been found a culture without some sort of religious belief system, it would be highly unlikely to find a mentally ill person without some sort of religious belief. So, this statement seems particularly biased.
On the other hand, a 22 year study suggest that aggressive atheism, characterized by debate and the need to belittle others, shows a strong correlation with SEVERE mental illness:
“Overt atheism, in other words, atheism that is professed as a way of life by some individuals — a manifesto of sorts — was found to be a common component inherent to many people suffering from other forms of severe mental illness,” said Dr. Hans Zimmermann MD PhD
“We started to notice that many of our experimental volunteers — those who were defiantly atheistic in their beliefs — were also suffering from various forms of dissociate disorders and social adaptation manias. This came as quite a shock to us as we studied patient after patient because for years we have been led to believe that religious zeal was often a patently obvious sign of mental illness, but in fact this is not the case. Quite the contrary.
“The individuals we have studied, especially those under the age of forty who actively participate in open debate about their own avowed atheism, or people who are very strident about their atheism are heavily prone to mental illness — and most of the illnesses are quite severe. This was not a handful of subjects who were studied, mind you. There were 17,809 people in the study. It was a very large model as far as psychological experiments go.”