Back in the day, before people knew about mental illness, they often blamed it on demons, spirits et cetra. Therefore, I contend that religion IS a form of mental illness.
So, you wish to adopt a position on mental illness justified by appeal to the medical expertise of superstitious ignorance of antiquity, the middle ages, etc.? "Before people knew about mental illness, they treated people appealed to religious notion. Therefore [i.e., using this logic] I contend we should do the same type of thing". Great.
Yes, it was a POSITIVE form of mental illness, but it was mental illness all the same.
I've had the DSM-IV TR for years, the ICD-10 for a while, and the DSM-V since it came out. I don't recall "religion" being in any of these or any other diagnostic manuals.
We are often taught to address mental illness as negative.
That's because illnesses are negative. However unscientific Freudian nonsense was, the basic idea behind separating pathology from just experiencing human emotions, reactions, etc., remains sound enough. Going through diagnostic manuals, one can diagnose themselves with practically anything. Freud simplified (overly, of course, but it remains apt) the central distinction between clinical distress and the normal ups and downs of life to one's ability to "love and work". Basically, if you find that you can't do the things you were able to do, or get no satisfaction or happiness that which you used to, or can't function in society because e.g., you can't adequately separate hallucinations (visual and/or another modality), etc., then this is "negative".
Put simply, a central factor in distinguishing mental illness for normal human experience
is negative effects. This is true even when it comes to would-be psychotic "symptoms" (in certain cultures, hearing the voices of recently deceased loved ones is not a clinical symptom but a social norm).
to portray what you saw as visions given by God.
Same with the use of substances, various methods used by Shamans (not in the limited Siberian sense but as the term is used more generally) to induce ecstatic states using everything from meditation to pain (e.g., hanging from hooks buried in their flesh), epileptic and similar neurological disorders, and normal human experiences given the right contexts. Before
Sybil, the number of cases of multiple personality disorder was practically nil. Thanks to a combination of the recovered memory movement and some popular media accounts of a (as it turned out later, untruthful) case study in particular, the number of cases boomed. A small number of mental health professionals were responsible for an extremely high number of diagnosed cased. One of the main reasons for this, it turned out, is that the methods used to determine whether or not patients had "alters" actually tended to create them, along with false memories, in people, particularly those more prone to suggestion. As another example, consider the Stanford prison experiment. A bunch of participants sign up for a study, are divided into "prisoners" and "guards" at random, the prison is a floor in Stanford with some additional trimmings for dramatic effect, and in a surprisingly fast time the prisoners were planning a revolt and suffering from a variety of symptoms (cognitive, emotional, physical) typical of actual incardination, the guards meanwhile began to turn into militant, power-hungry sadists. Finally, the PI heading the study stopped it because his research assistant (later wife) told him it was out of control and beyond dangerous.
Some people might think that is true, but personally, I don't. I believe they were visions given by God and meant for people to understand.
You're talking about a VERY select type of extremely small symptom manifestations among a small subset of those diagnosed with mental illness whilst ignoring how many individuals can have these same symptom manifestations due to other factors.
Do you acknowledge that it was thanks to the gifts these people possessed that we have the institutions and scriptures we have today ?
No, I think its patently ridiculous. Possessed people weren't believe to be speaking for god, and even those who were thought blessed because of glossolalia couldn't possibly be responsible for scriptures because, well, they weren't speaking any language.