Illness or what?
There is a sharp difference between biological/organic induced illness as it affects one's mental capacity and learned/self-induced behavioural problems.
The organically-induced illness may come our way without asking for it. If, quite by accident, I bang my head against a wall, I may suffer a mental illness, properly and truly so-called. I might fall into a coma - my mind will not function the normal way, to such an extent that I cannot go about my daily work.
In the same way, if I take toxics into my biological system, the brain may be affected and I fall sick, mentally speaking. A person suffering for shortage of oxygen for a long time may also have his brains damaged. Due to complications during pregnancy, a child may be born with a mind that cannot function properly.
On the other hand, and in a totally different category, self-induced behavioural problems have nothing to do with the above. A so-called manic-depressive may have cheated for so long and ran away from his responsibilities that he has now developed a mental "condition," - very natural to him - so much so that it appears to be a sickness. But in reality he brought it upon himself. He is the weak prisoner and the tyrant jailor all at one go. He is the victim, but the one dominating him is himself. His self-pity, self-love and pride has brought this condition upon him. As he developed it, he can by God's grace get out of it and find liberty.
Similarly a man suffering from amnesia has nothing wrong with his memory. His sinful behaviour has been such that he has eventually found it intolerable to cope with society. So he retreats from reality; he disassociates himself from everything and everyone else. He says: "I will have nothing to do with that wicked me." This shows up in his amnesia: "I will forget that he exists or ever existed."
There's nothing wrong with the schizoid, biologically speaking. His pride and self-centredness induce him to withdraw himself from those who will not give him what he wants and desires. He says: "They are not worthy of me," so he dismisses them and despises them.
For the first type of illness, we need help and compassion. For the second type we need to be brought to repentance. The first type is biological; the second type has a moral and spiritual root, namely, sin.