I have a trivia question for you: What is the first public act of ministry Jesus performed? Answer: We don't know. Each gospel shows a different first act.
Matthew places Jesus in a teaching role, opening up his public ministry with the Sermon on the Mount (Matthew 5-7). Luke wants us to know that in Jesus the needs of the oppressed will be met as he fulfills prophetic anticipation (Luke 4:16-21). John shows his power through the miracle at the wedding at Cana (John 2:1-11). Mark 1:21-28, the gospel lesson for Sunday, shows Jesus teaching and expelling a demon as part of the same scene, establishing his authority among us.
The people were amazed, but it will take a while for them to realize what the demon already knows - that Jesus is the Holy One of God and has come to wage war against their power, a war whose outcome, the demon knows, has already been decided.
If you're wondering which gospel-writer's presentation of Jesus' first act is correct, you're stuck in the trivial. Remember, these are not biographies in the sense that they lay out a step-by-step chronology of Jesus' life. These are expressions of the good news of God's merciful presence among us in Jesus Christ. To that end, they share what they find most significant in the order that best presents the gospel.
Mark was convinced that to tell the story he needed to tell, it should start with a bold announcement that a new authority had arrived, one who will challenge scribes, demons, and anyone or anything that dares to oppose God.
Mark will rush us to the final battle scene at Golgotha, but the demon's response in the first scene is telling. He knows what Mark wants us to know - that the arrival of the Holy One of God means the war's outcome has been decided and those who are on the side of God will be pleased with the outcome.