Jordan,
You are asking a question that vexes people like no other question that I know.
What kind of mental disease are you talking about? Alzheimer's? I personally know of an evangelist, Bro. James, who was used mightily of the LORD who developed this disease. As this disease progressed in this man, his speech got awful, foul, vile. One preacher told us how he visited him and wished he hadn't: the foul language that spewed from that man's mouth was sickening. Bro. James was a very wicked man in his youth and well into his marriage, but was soundly converted and used mightily of the LORD in his kingdom. How could this happen? It vexes people to no end.
My husband did alternative service (to military service)in a mental hospital in the 60's. The conclusion of the Christians who worked there was that a large number of the patients were there because of sin.
We have a friend whose son in presently in a mental hospital. His mother declares that if he would submit to authority, submit to God his problems would vanish.
Jordan, I am not sure there is a definitive answer to your question. This may be a realm that belongs to God to judge and we must leave it there. Consider 2 Cor. 5:13: "For whether we be beside ourselves, it is to God: or whether we be sober, it is for your cause." According to Strong's the term "beside ourselves" refers to being insane.
The best we can do, IMO, is make sure we walk with Him and live in the center of his will and make sure ALL of our sins are under the blood. And then commit our future to His care and keeping.
My thoughts...what are yours?
ginnyrose