Exorcism can be dangerous, Jesus said, because it can leave you worse off.
“When an evil spirit comes out of a man, it goes through arid places seeking rest and does not find it. Then it says, ‘I will return to the house I left.’ When it arrives, it finds the house unoccupied, swept clean and put in order. Then it goes and takes with it seven other spirits more wicked than itself, and they go in and live there. And the final condition of that man is worse than the first. That is how it will be with this wicked generation.” – (Matthew 12:43–45)
The problem in this scenario is that the soul of the exorcised person is not filled with another and better spirit. It is merely “swept clean.” It may be “put in order,” temporarily following a set of rules for moral living. But it is vulnerable to spiritual attack. This is the condition of so many who attempt to become morally good by following rules and strict discipline. In Jesus’ day, it was the Pharisees who followed that path. In our day it is frequently those raised in a legalistic church who find themselves in this kind of peril. His or her “house” is “swept clean” but it is “unoccupied.” Take that person out of their childhood environment and plunk them down, unsupervised on, say, a college campus and some very strange and sad things tend to happen.
However, when a person trusts Jesus, He sends His Spirit to live in their soul to guide them and empower them in truth.
And I will ask the Father, and he will give you another Counselor to be with you forever— the Spirit of truth. The world cannot accept him, because it neither sees him nor knows him. But you know him, for he lives with you and will be in you. – (John 14:16–17)
But when he, the Spirit of truth, comes, he will guide you into all truth. – (John 16:13a)
This is an essential difference. Their “house,” their soul, is no longer “unoccupied.” It is the reason why the message of Jesus is not merely another religion, doomed to failure, but is genuine, Good News! He gives the Holy Spirit Who lives in our souls and overpowers the forces of evil.