From The Gospel of Mark, by Mary Healy, commenting on Mark 1:21-28
Jesus’ teaching has the intrinsic effect of exposing evil so that it can be expelled. Mark does not explain whether the man with an unclean spirit was a regular synagogue attendee or whether he came specifically to disrupt Jesus’ sermon. But in the presence of Jesus, the grip of evil on the man comes to light and he cries out in fear and rage, What have you to do with us?
The spirit is challenging Jesus’ encroachment on the demons’ formerly uncontested territory, evidently aware that his coming portends their downfall. The spirit claims hidden knowledge of Jesus’ identity, a frequent demonic tactic (3:11; 5:7) that may be intended to catch Jesus off guard or gain some control over him. But the attempt is futile.
“Holy One” is a term usually reserved for God (1 Sam 2:2; Hosea 11:9) but is occasionally used for those who are consecrated in his service as priests or prophets (Num 16:5–7; 2 Kings 4:9; Ps 106:16). Holy One of God is an accurate title for Jesus (see John 6:69), but not one that he wants publicized at this point in his mission. He will reveal his identity on his own terms and in his own time, to ensure that it will be rightly understood.
Jesus sternly rebukes the spirit: Quiet! (literally, “Be muzzled!”) Come out of him! In a final show of defiance, the unclean spirit convulses the man as it departs, helpless before Jesus’ word of command. Already the Baptist’s prophecy of a “mightier one” to come (v. 7) is being fulfilled before the people’s eyes. The demon’s tyranny is over and the possessed man is set free.
The people react with amazement: What is this? A new teaching with authority. They recognize an intrinsic connection between Jesus’ teaching and his power to dispel evil.