What does Mark 1:24 mean?

Being a spirit, the demon knows information not yet revealed in the physical world. Anyone there could have known that Jesus was from Nazareth, but few would have made the connection with Isaiah 9:1 that prophesies the Messiah will come from Galilee—the district of Nazareth and Capernaum. But the fearful questions "What have you to do with us?" and "Have you come to destroy us?" could only come from a being who really knew Jesus' identity—which the demon affirms—and has reason to fear.

The destruction of demons is an unclear subject in the Bible. The Greek root word used here, apollumi, can mean "to render useless, kill, or send to eternal misery in hell." After the crucifixion, Jesus will preach to the "spirits in prison." These are apparently demons from before the Flood who sinned so grievously God imprisoned them (1 Peter 3:18–20). These demons may also be the locust horde that will be released during the tribulation (Revelation 9:1–6). Apparently, the demon understands that if He wants to, Jesus can banish it to the same place.

Though those in the audience are still wondering about Jesus, this evil spirit knows Christ's identity. Jesus is the Holy One of God (Luke 4:34; John 6:69). This "holy one" was noted by the psalmist (Psalm 16:10; 78:41; Acts 13:35) and was a term used of God. This spirit recognizes Jesus as divine (1 John 2:20; Revelation 3:7; 16:5).