I think the whole thing of demon possession is somewhat mysterious.
Does the Bible ever even use the expression demon possession?
Is it just sometimes that those who have an underlying physical condition such as depression or epilepsy, are more open to the suggestions of Satan because they have weak minds?
E.g. someone who has depression may be open to the temptation which may often be of the Devil rather than the individual to kill himself. But once the underlying condition of depression is treated, this temptation goes away. Some with mental illness may be tempted by the devil to harm or kill others, whereas if the underlying condition was treated the Devil would find no success or foothold.
Is "demon possession" sometimes just that people are subject to successful demonic attack because of underlying conditions?
May be sometimes its the other way around, and the demon is causing the condition.
I certainly believe that occult involvement can lead to people "having a demon", but it doesn't always manifest itself as one might think.
Knox Free Church,
Perth, Scotland GB
His Name forever shall endure;
last like the sun it shall:
Men shall be blessed in Him,
and blessed all nations shall Him call (Ps. 72:17)