In the synagogue there was a man possessed by a demon, an impure spirit. He cried out at the top of his voice, “Go away! What do you want with us, Jesus of Nazareth? Have you come to destroy us? I know who you are—the Holy One of God!” “Be quiet!” Jesus said sternly. “Come out of him!” Then the demon threw the man down before them all and came out without injuring him. All the people were amazed and said to each other, “What words these are! With authority and power, he gives orders to impure spirits, and they come out!” –  Luke 4:33-36 NIV

Well, I think this belief of a demon inside him might be a little different than what it really was. I think it was not a demon, but a person who is stressed and out of his normal sense of being. Back in those days they may have thought there was a demon inside the person, but now we know it is stress. Basically, he healed the person and there was no demon inside of him. What is called stress now, was known to be a demon back in those days.

Stress 

As over 2 thousand years have passed by, we have educated ourselves and now know what we didn’t before. Stress starts in a person when something is bothering him/her. This stress causes the body to function differently which is out of the normal state of being. Back in those days people thought it was a demon, until now we are aware that it is stress and not a demon.
I may be wrong with what I am saying, but I’m sharing my knowledge to what I think it is. I feel like it would be great if Jesus came again to heal us all again.