Matthew 12:43-45  As Jesus completes his discussion with the scribes and Pharisees he tells a parable about a demon possessed man whose demon leaves for a while, wanders around, and returns.  In the meantime, the demon's "house" has been swept, cleaned, and put in order.  He is so thankful he goes and brings 7 worse demons with him.

This is not a story about all people are demon possessed, or the ineffectiveness of exorcism, or any such thing.  It is a graphic depiction of sin and the effects of sin on people that refuse to replace the sin in their life with the Spirit.