Actually, father Matt, I would correct this. Mental illness, in my opinion, is not only biological, although certainly I believe a biological cause can produce it. We are talking specifically about mental, what people think, and so, mental error, being genuinely wrong, thinking wrongly can produce a chain effect that leads to incorrigible error. The sexual insanity we see today, for example, is, I believe, both spiritually produced by desire and passion, encouraged by demons, AND error about what is good, right, and true. In other words, we wind up buttressing our wrong desires by wrong arguments for them. I have said a thousand times here that wrong language is the beginning of error in thinking about things; in the beginning was the word. Saying that a person “is gay”, “has a sexual orientation” (rather than a passion), that his sex is actually his “gender”, are the beginnings of that chain of error that lead to full-blown insanity.
This can be expanded into anything, not only sexuality, of course, as a Christian can go mad over concern about climate change, beginning from the right idea that we are supposed to be good stewards, and going wrong by trying to make it a major concern of the Church, even above and beyond “the Great Commission”, and attempting to force everyone to be good stewards by secular schemes through force of law. From this comes “Earth Day” and other modern idiocies that help us imagine that we can be our own gods and “save the world” through our own efforts. That goes for mad ideas like “reparations” as now being promoted for ancient sins by and against people long dead, the idea that we can do what only God can - impose historical justice. All are instances of mental failure begun in wrong thinking, by no means necessarily biological, but simply in believing something that is false.