What does Mark 9:25 mean?It isn't clear why the appearance of a crowd accelerates Jesus' actions. They are already surrounded by a great number of people (Mark 9:15), but more are coming. Unlike with the deaf man (Mark 7:33) and the blind man (Mark 8:23), there is apparently no time to draw the boy away.
We don't exactly know where Jesus and the disciples are. They are seven days out of Caesarea Philippi which is far north of Bethsaida and the Sea of Galilee, and one day away from a tall mountain (see Mark 8:27; 9:2; Luke 9:37). They will have to travel through at least part of Galilee to get to Capernaum (Mark 9:30, 33). But His intentional, public ministry in Galilee is over (Mark 8:12–13), and for quite some time, He has tried to find quiet places to teach the disciples, even as He willingly heals those who cross His path.
So, it's likely that Jesus cuts short His lesson on faith to take care of the demon before yet another crowd can mob Him (Mark 3:7; 4:1). Although it would do the crowd well to learn more about having faith in God, it is more important that Jesus have the opportunity to teach the disciples what they need to know to develop the church. A quick object lesson as He expels the demon will have to do.
This may be the only time Jesus expels a demon and demands that it not return. He has made it clear that a person who is freed from a demon can expect the demon to reappear with other spirits (Matthew 12:43–45). The only permanent cure is for the person released from demonic possession is to accept the Holy Spirit. But it's safe to assume this demon obeyed Jesus' command.