What does Mark 9:20 mean?

The tenor of the demon's response to Jesus is normal, if the violence is unique. The demons in Capernaum (Mark 1:24–26; 3:11) and Decapolis (Mark 5:6–7) seem impelled to approach Jesus and react in a way that shows they know His power and authority and that He is their enemy. Where Peter, James, and John are afraid when they see a glimpse of Jesus' deity (Mark 9:6), the demons respond with hate. Most demons, upon seeing Jesus, are victim to a compulsion to declare who He is (Mark 1:24; 3:11; 5:7). This demon appears to make the boy mute because it is mute, and expresses its rage in more physical ways.

Ironically, as the demon acts out in the face of Jesus' authority and deity, the father interprets the violent display as evidence that Jesus can't control it (Mark 9:22).

For centuries, western culture has seen demons as titillating entertainment. Apocryphal and pseudepigraphal books, such as the falsely-attributed "Testament of Solomon," claim to classify and name particular demons. More recently, demons have been portrayed as anti-heroes, tragic victims, and romantic interests.

This account more clearly describes what demons are like: destructive and hateful, more than willing to kill a child (Mark 9:22). Demons do not deserve our pity or understanding. They do not have an opportunity for forgiveness, and they do not want one. They are the enemies of God, not plot-points for modern entertainment.