Mentall illness is a huge umbrella of terms. Have you been depressed? Well that's a mental illness. Have you had a cold? Well that's a physical illness.
There are levels, extremes, long term, short term, etc...to take on such a huge subject matter, there needs to be qualifications, because qualifying your statements so they are not misunderstood is important.
Does anyone remember some years ago when there was a group of people who believed they were a part of some cult that were to meet a mother ship of some kind (forgive me, I don't have the full record, in front of me), who got rid of their sexual parts, and then took poison based on the alert of some star around the sun?
There is something called mass hysteria, mass delusions, group dynamic studies show how people can be molded by distorted ideas and ideology and lead to irrational ideas about oneself and others.
In religion, it is very likely that people can be influenced to think irrationally based on unfounded and unqualified things which can lead to distorted behavior and distorted views. It is not an absolute, one has to learn to separate the levels of mental illness and underestand exactly what someone is saying.
I fear that whenever people discuss mental illness they relie solely on extremes such a schizoprenia, severe paranoia, etc when in actually people may have a bout of mental illness, get treatement and move on. I would continue but I am in the middle of something, but I wanted to contribute to this topic.