"A mental illness is a medical condition that disrupts a person's thinking, feeling, mood, ability to relate to others and daily functioning. Just as diabetes is a disorder of the pancreas, mental illnesses are medical conditions that often result in a diminished capacity for coping with the ordinary demands of life."
Is Religion a mental illness? The religious make all kinds of wild claims without a shred of evidence. It is solely based on wishful thinking. Religion makes the most absurd and arbitrary demands of its followers. Religion threatens eternal punishment or death if those demands are not followed. Last but not least it's followers show utter contempt for rational thinking and a never ending urge to impose itself not only on believers, but also onto nonbelievers.
Religion is a confidence scam intended to control, manipulate and exploit people that have been conditioned from birth to believe in the unbelievable. This should be painfully obvious to anyone capable of even the slightest amount of critical thinking. Despite the tendency of people to be suspicious and critical in most aspects of their lives, it still seems inexplicably that these standards of proof get thrown out the window when applied to religion.
Just because a delusion is a mass delusion doesn’t make it any less of a mental illness.