What does Mark 9:26 mean?
The reaction of the demon to Jesus' authority is normal, even for those without seizure-like manifestations. In the beginning of Jesus' ministry, when He heals a possessed man in the synagogue, the demon, "convulsing him and crying out with a loud voice, came out of him" (Mark 1:26). The legion of demons caused such violence they destroyed a herd of pigs (Mark 5:13). When Philip exorcised demons in Samaria, they came out with a loud cry (Acts 8:7). Since this demon is mute (Mark 9:17), he expresses his rage physically.Demons are former angels who rebelled against God (Isaiah 14:12–15; Ezekiel 28:6–9). They rejected their duty to glorify God and followed Satan in his quest to seek his own worship (Revelation 12:3–4, 9). They are not our friends. The power they offer is meant to destroy us. They have no power or authority over God, but they can take away from His worship by tricking, leading astray, and attacking people. The violence they exhibit when leaving a person shows that they are malevolent to the end.
Apparently, the deathlike appearance of the boy is a normal result of a seizure. "Becomes rigid" in Mark 9:18 can also mean that the boy is exhausted, withered, and pale. The physical stress of a seizure naturally makes the boy appear dead, but the members of the sudden crowd (Mark 9:25) wouldn't necessarily know that.