Acts 19:11-20

11God did extraordinary miracles through Paul, 12so that even handkerchiefs and aprons that had touched him were taken to the sick, and their illnesses were cured and the evil spirits left them.

13Some 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." 14Seven sons of Sceva, a Jewish chief priest, were doing this. 15(One day) the evil spirit answered them, "Jesus I know, and I know about Paul, but who are you?" 16Then 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.

17When this became known to the Jews and Greeks living in Ephesus, they were all seized with fear, and the name of the Lord Jesus was held in high honor. 18Many of those who believed now came and openly confessed their evil deeds. 19A number who had practiced sorcery brought their scrolls together and burned them publicly. When they calculated the value of the scrolls, the total came to fifty thousand drachmas. 20In this way the word of the Lord spread widely and grew in power.

Miracles, Magic & the Gospel

These Jews traveled from town to town making a living by claiming to heal people and drive out demons. Often they would recite a whole list of names in their incantation to be sure of incluing the right deity. Here they were trying to use Jesus' name in an effort to match Paul's power.
Also many Ephesians engaged in exorcism and occult practices for profit. The sons of Sceva were impressed by Paul's work, whose power to drive out demons came from God's Holy Spirit, not from witchcraft, and was obviously more powerful than theirs.

The power to change people comes from Christ. It cannot be tapped by reciting his name like a magic charm. God works his power only through those he chooses.