Acts 19:13-16 (NIV)
Some Jews who went around driving out evil spirits tried to invoke the name of the Lord Jesus over those who were demon-possessed. They would say, "In the name of Jesus, whom Paul preaches, I command you to come out." Seven sons of Sceva, a Jewish chief priest, were doing this. One day the evil spirit answered them, "Jesus I know, and I know about Paul, but who are you?" Then the man who had the evil spirit jumped on them and overpowered them all. He gave them such a beating that they ran out of the house naked and bleeding.
I could not help but laugh as I was reading this. The official Apostles (messengers) of the early church had special gifts including that of being able to cast out demons. So, you have these priests who are watching all this going and they decide to get in on the action of driving out evil spirits. The words they use belie there motivation, "In the name of Jesus, whom Paul preaches..." Paul preaches Jesus, and they evidently don't. They are just attempting to use his name for their own purpose. What happens next is funny: they chance upon a real demon spirit who confronts them with "Jesus I know but who are you?"
Beyond the obvious humor in this story there are some interesting insights we can gain into the spiritual world. First, the evil spirits are well aware of who the enemy is, they know of Jesus. Secondly, it is clear that it is not enough to simply know Jesus or believe in him to be saved; certainly the demons aren't saved. (Faith: trusting in Jesus alone for eternal life is the way of salvation) In the book of James, it says "even the devils believe and tremble." Thirdly, there must have been a great deal of demonic activity during the growth of the early church. This stands to reason since there would be a motivation to nip the church in the bud before it gets too large. Fourthly, we don't see much of this "casting out demons" today.
What is the purpose of miraculous gifts such as casting out demons and healings? The Bible always speaks of them as a sign. They authenticate, corroborate, and confirm the message given by the early church leaders. The pattern of the book of Acts which happens over and over was that one of the leaders of the church like Paul or Peter would go into a city, perform a miracle, and then preach the message which many times contained new revelations from God. Had it not been for the miracles their controversial message would not have been believed.
Is there a need for this today? You could argue yes, we need to know whether this person speaks for God or not. Well, think again, if we have the whole revelation of God in our Bibles today, then to know if someone speaks for God, we just need to compare what he says with the Bible. If it matches up, then he is speaking God's word if it does not then we can safely dismiss him. This makes sense to me and it fits with what the Bible says about these things.