When a person becomes insane, what is the cause? Are all such happenings because of demon possession?
Demon possession can and has resulted in individuals becoming mentally unstable. The former King Saul became mentally disturbed by an evil spirit or demon, according 1 Samuel.
Now it came about on the next day that an evil spirit from God came mightily upon Saul, and he raved in the midst of the house, while David was playing the harp with his hand, as usual; and a spear was in Saul’s hand. (NASB) 1 Sam. 18:10
In 1 Samuel 15:26 we are told that King Saul ceased to be the king of Israel after he disobeyed God. Shortly after that God removed the Holy Spirit from Saul (1 Sam. 16:14). At that point Saul did not have the anointing of spiritual power to minister for God. This is a serious warning to anyone who seeks to serve God in his or her own way. God demands that we minister as He desires and with holiness. If we do not, God will eventually use someone else and withdraw the spiritual empowerment that comes through the Holy Spirit. When we come to 1 Sam. 18:10, we discover that God now allows an evil spirit to control Saul. The Hebrew word that is translated as “raved” in the passage (1 Sam. 18:10) above is actually “prophesied.” That is, the evil spirit was able to speak through him. 1 Sam. 16:23 tells us that Saul would be refreshed or become pleasant again when David would play his harp. Demon possession caused Saul to be mentally unstable.
So it came about whenever the evil spirit from God came to Saul, David would take the harp and play it with his hand; and Saul would be refreshed and be well, and the evil spirit would depart from him. (NASB) 1 Sam. 16:23
Mental instability can be caused by different things. However, demon possession is not possible for a Christian because the indwelling Holy Spirit is more powerful than any evil spirit.
You are from God, little children, and have overcome them; because greater is He who is in you than he who is in the world. (NASB) 1 John 4:4
The evil spirit did not come upon Saul (1 Sam. 16:23) until after the Holy Spirit had departed (1 Sam. 16:14). Every Christian should remember that the Holy Spirit indwells us until we die or are raptured and go to meet our God. Mental instability cannot and will not separate us from our God. One who is truly saved is going to heaven, period. May the Lord bless you.
Reference Links:Saved Only Once
Can Christians be demon possessed?
Is our spirit at the right hand of God?
Angels - Good & Evil