Jesus commands the demon to leave, giving it a short, direct order backed by God's authority. He does not rebuke the man, because the unclean spirit had possessed him, yet each of us must resist the influence of demons (I Peter 5:8-9). Jesus tells the demon, "Hold your peace," which actually means "be gagged or muzzled," a phrase He also uses to calm the storm in Mark 4:39. The unclean spirit does not speak again, but obeys in rage and anguish.
By his own power or authority, no man can cast out demons. Even the archangel Michael, not daring to revile Satan, called on the power and authority of God to rebuke him (Jude 9), setting a right example for us. Similarly, in rebuking the "spirit of divination" at Philippi, Paul says, "I command you in the name of Jesus Christ to come out of her" (Acts 16:18).
Because of Christ's authority in performing this miracle, the people in the synagogue are "amazed," a word meaning "to stupefy" and "to dumbfound or flabbergast." They express their astonishment in questions: "What is this? What new doctrine is this?" (Mark 1:27), as well as by immediately rushing away to tell everyone they can. The word translated "amazed" also can mean "to terrify" and "to be frightened." The people are not only astounded but also fearful of God's power through Jesus.
The focus of the testimony is on how Jesus exorcises the demon: simply by His command, which shows the power of God's Word. Contemporary Jewish doctrine for casting out demons was much different, as exorcists among them sometimes appeared to cast out demons by prayers or chants. Christ, however, does not cajole or request demons to leave, but authoritatively commands them to come out. The world has its weak and useless methods to appease evil and entice it to surrender, but Christ commands its defeat.
Martin G. Collins
The Miracles of Jesus Christ: Exorcism in the Synagogue