Mark 1:23-28 : The first victory over the powers of evil

In synagogue , while Jesus is teaching, a man with evil spirit shouts out ‘What have we to do with you, Jesus of Nazreth ? Have you come to destroy us ? I know who you are.’
Jesus immediately chastises the spirit and orders it to come out of the man. The spirit immediately leaves the man. People are dumbstruck that he has authority over spirits too.

This brings us to the question of ‘how/why do spirits exist ?’. Jews believed that spirits were descendants of angels and humans. The story goes this way. There were 2 angels – Assael and Shemachsai who forsook God and came into earth attracted by beauty of mortal women. After sometime one of them repented and went back to heaven while the other remained here to gratify his lust. Demons are the children he begat and their children. Their main aim is only to do evil. (a note about descendants of heavenly and earthly beings in given in Genesis 6:1-8).
Whatever it is evil spirits did/do exist. Probably it is another way of telling about an MPD(Multiple personality disorder). But since the victim believes that it is real , the cure is to make the person believe that spirit is gone out of him in the reality even if it means to just clear his thoughts.