The witnesses were amazed at His authority. They were amazed at His words before, and now at His works. The whole area heard about this. I need to believe that Jesus possesses inherent power because of who He is. Amazement is not enough.
Faith is essential.
All the people were amazed, but not all believed. He is teaching in the synagogue. The devil comes to church too. We often think that if I can just go to church, I'll be safe there. From the emphasis in the passage on authority and the demon's reaction to Jesus, we see that Satan is active in attacking the authority of Christ.