What does Matthew 17:18 mean?

As did many ancient writers, Matthew often skips details and condenses accounts. While he seems especially detailed in his records of Jesus' words, he more often summarizes Jesus' interactions and miracles. He describes the exorcism of the demon from this boy (Matthew 17:14–17) very simply: Jesus rebuked the demon. It left, and the boy was instantly healed of the symptoms the demon had been causing. Mark provides much more information about this incident (Mark 9:20–27). When the demon, from inside the child, sees and recognizes Jesus, it causes the boy to go into a convulsion and fall to the ground, rolling around and foaming from the mouth.

The father asks Jesus to have compassion and help "if" He is able. Jesus does not let the doubt implied in that statement pass. Doubt is the very thing which has caused Him so much exasperation during His ministry (Matthew 17:17). He tells the father that all things are possible for the one who believes. The father famously responds by saying "I believe; help my unbelief" (Mark 9:24). This has become the prayer of many people down through history struggling with faith in the face of their limited understanding.

Jesus quickly casts the demon out of the boy by saying "You mute and deaf spirit, I command you, come out of him and never enter him again" (Mark 9:25). The demon convulses the boy once more and then leaves. The boy appears dead, but Jesus takes him by the hand and the boy stands up.