There will always be people who try to use the kingdom of God for their own ends. These sons of Sceva are a case in point. First of all, it might help to know that there never was a Jewish high priest named Sceva. So from the get-go, these guys are frauds, or their father was a fraud, or something.
Next, we see these guys not caring about the substance of Paul's message, but merely about its effectiveness. In other words, they want to use it for themselves. They make their living by casting out demons, and Paul seems to have a better mousetrap. So the next time they have opportunity, they confront a demon by saying, "In the name of the Jesus whom Paul preaches ..." How foolish! How idiotic! They don't have a clue what they're messing with, but they want to use the name of Jesus because it works.
The demon's response is fascinating as well. "Jesus I know, and Paul I have heard of, but who are you?" This has to be one of the best comic lines in all the Bible. Unpack it. First, the demon knows Jesus. Doesn't obey Jesus, doesn't worship Jesus, but knows him. (See James 2:19) Second, the demon has heard of Paul. In other words, if you begin to make an impact for God's kingdom, the spiritual world is going to pay attention to you. In fact, the Bible says that one of the main reasons God started the church was to teach the spiritual world about his grace and mercy (see Ephesians 3:10). Third, you can't use Jesus for your own ends. It won't work. He won't serve you. You can hide behind him and deceive people in the short term, but long term you'll be found out.
Finally, look what happens to these shysters. The demon-possessed man beats them all up, and leaves them naked and wounded. Is it just possible that this is exactly where God wants these guys -- naked and wounded? God is close to the broken, the hurting, the exposed. And they are close to him. If they will turn to him in that broken moment, he will receive them, heal them, and reveal himself to them. This is the good news!
I'll leave you to read what happened after -- how God used this incident to move people to repentance! Also, take a look at Ephesians 6 and see how Paul continued to lead the church in Ephesus deeper into the truths of the spiritual world.