There's so much in this one chapter. I had a hard time picking out just one concept. V 1-7 talks about how Paul came across some disciples, from the outside they seemed like Christians, they thought of themselves as Christians, but of course Paul was so in tuned with God he knew they needed the truth. They were baptized with John the Baptist baptism but not by the Holy Spirit. Paul explained and they accepted and the Holy Spirit filled them. Then in v 13-16 some took it upon themselves to cast out demonds. They saw Paul do it and thought they could too. They called them out in Jesus name and Paul's. What I find interesting was that the demonds talked to them and told them they knew who Paul was and knew who Jesus was, but didn't know them. They came away beaten and stripped. Wow, powerful lesson. We have no earthly idea how strong the spiritual world is around us. We also have no earthly idea if the person next to is or who we see everyday knows our Heavenly Father and has the Holy Spirit living in them unless we ask. Lesson learned for me.