Instantly it left her ...
One day as we were going down to the place of prayer, we met a demon-possessed slave girl. She was a fortune-teller who earned a lot of money for her masters. She followed Paul and the rest of us, shouting, “These men are servants of the Most High God, and they have come to tell you how to be saved.” This went on day after day until Paul got so exasperated that he turned and said to the demon within her, “I command you in the name of Jesus Christ to come out of her.” And instantly it left her. -Acts 16:16-18 NLT
From the very beginnings of my walk with the Lord I have been around people who practiced exorcism of demonic forces. Regularly I have seen ministers pray for people trying to cast out demons. Mostly what I saw was religious in nature. People would practice and say what they heard and saw other people doing. And on a very rare occasion the person would be helped. Sadly in most cases, the prayers were ineffectual because they did not involve discernment.
This passage from Acts is very different from those experiences. It took Paul a while to discern that this girl was troubled by a demon. After all, she was giving testimony about them and how they had come to offer salvation. Yet something happened that day and Paul discerned the demon - and in an instant it was gone as Paul spoke the name of Jesus. The example I glean from this account is that discernment, not religious repetition, is required to deal with evil.
Deliver us from evil Lord and help us to discern how to deal with it when it comes.