17:14–20 When Jesus, Peter, James, and John descend after the transfiguration, they see a crowd. From it a man emerges, kneels before Jesus as a supplicant,277 and pleads, “Lord, have mercy on my son, for he has seizures and . . . often he falls into the fire, and often into the water” (v. 15). “Has seizures” translates selēniazomai, which appears only here and in 4:24 in the NT.278 As usual in the Gospels, the term describes symptoms, not a specific disease—the boy suffers seizures that have a self-destructive bent. He does not fall at random but into fire and water. Whatever the biological root, demonic activity compounds it.

In Jesus’ absence, the father had brought the boy to Jesus’ agents: “I brought him to your disciples, and they could not heal him” (17:16). The hint of accusation is stronger in Mark: “I brought my son to you [the Greek is singular, referring to Jesus]. . . . I asked your disciples to cast it out, and they were not able” (Mark 9:17–18). If the father stresses the disciples’ inability to act in Mark 9, Jesus notices their faithlessness in Matthew 17. He calls them a “faithless . . . generation” in verse 17, and in verse 20 he remarks that they could not cast out the demon because of their “little faith.” The disciples’ lack of progress is a dominant theme throughout chapters 14–17. The disciples forgot about Jesus’ powers (14:16–21; 15:33), panicked and doubted (14:26–31), misconstrued Jesus’ teaching (15:15–16; 16:4; 17:4), and drove out a seeker (15:23).

The boy’s father views Jesus’ disciples as his representatives (17:16). In the absence of the Master, deputies should do the Master’s work. Indeed, they have healed disease and cast out demons in the past (10:8; Luke 9:6; 10:17). But a demon stands behind the disease, and he resisted them successfully.

This event offers a glimpse into Jesus’ emotional life. Their faithlessness exasperates him: “O faithless and twisted generation, how long am I to be with you?” (Matt. 17:17), expressing the weary vexation that is clear elsewhere in Matthew (12:39; 16:4) but especially sharp in Mark (3:5; 8:12, 17–21). Jesus has lived with the faithless for thirty years, and his disciples’ apparent degeneration distresses him. “How long am I to be with you?” also suggests that Jesus had an existence prior to his incarnation and desires to return to it.

Laying aside his disappointment, Jesus heals the boy and expels the demon by his command. The disciples wonder, “Why could we not cast it out?” (Matt. 17:18–19). This question may indicate surprise at their failure—after all, they have handled similar assignments. What went wrong?

Jesus replies, “Because of your little faith” (v. 20).279 Alas, his answer prompts additional questions. Have the disciples regressed? Do they rely on past gifts and experiences instead of present faith? Or is this a stronger demon (if they have ranks) whose expulsion requires more power?

This passage provides a window on divergences in the Synoptics. Matthew explains the event through his distinct interest in the “little faith” of the disciples. His term is oligopistian, used only here in the NT but close to his almost unique term for the Twelve, “little faiths” (oligopistos, used outside Matthew just once). Mark also makes faith central, but he focuses not on the disciples but on the father as he comes to faith. The father’s confidence has been shaken by the disciples’ failure to heal his boy. Therefore, when he meets Jesus, he begs, “If you can do anything, have compassion on us and help us” (Mark 9:22). Jesus notes the insecurity and quotes his words back to him: “‘If you can’!” To paraphrase, “What do you mean, ‘If you can’?280 The issue is your faith, not my power, for all things are possible for one who believes” (Mark 9:23). The father’s reply is classic: “I believe; help my unbelief!” (Mark 9:24). The theme of coming to faith with difficulty also appears in Mark 8:22–26. The accounts of Matthew and Mark are complementary; each attends to the difficult path to faith. Mark emphasizes the journey of outsiders, while Matthew focuses on that of the Twelve.

Jesus concludes with encouragement. Even if the disciples have a little faith, it is enough, since the faith of a mustard seed, the tiniest of cultivated seeds in that culture, can uproot a mountain and move it “from here to there.” With faith, nothing is impossible (Matt. 17:20). Moving mountains is a metaphor for the power of God, or the faith to do what seems impossible (cf. Isa. 40:4; 49:11; Mark 11:23). This is not raw power to do whatever one wishes but power to do what God wills and authorizes.