MARK 1.27-28: And they were all amazed, so that they questioned among themselves, saying, “What is this? A new teaching with authority! He commands even the unclean spirits, and they obey him.” (28) And at once his fame spread everywhere throughout all the surrounding region of Galilee. (ESV)
When Jesus exercised His authority over this demonic spirit, things in the synagogue changed. No one had ever seen anything like this. The Bible indicates that there were people at that time who believed in the reality of unclean spirits. Examples in the New Testament of people expressing this very thing can be found. The Syrophoenician woman of Mark 7.24-30 is an example.
But Jesus actually casting out a demon from someone was a new type of event in Israel. There is no record of any such thing being performed in the Old Testament, not even by Moses or Elijah. And the people of this particular synagogue recognized how profound this actually was.
What is interesting is that the greatest degree of attention and amazement was directed towards the teaching of Jesus. It was new, and the act of Jesus exercising His authority over the unclean spirit served not only to liberate the man from the demonic oppression, but also to authenticate the teaching of Jesus and His authority.
The teaching of Jesus was new in terms of its style in that it was based upon Jesus Himself, rather than a long list of teachers and their interpretations from years long gone by. Jesus’ teaching also had new content. The core message of Jesus was a call for people to repent and believe in the Gospel.
Before Jesus, the prevailing idea of how a person could be made in right standing with God was founded upon a person working to keep God’s Law. It was an effort seeking personal righteousness by means of good works. The Pharisees were foremost in this pursuit. For someone to repent would be an acknowledgement of failure and people did not particularly want to do this. And the call for people to simply believe in the Gospel didn’t have the ring of accomplishment that people seeking to earn their salvation would be attracted to.