As Moses found that those he had left in charge were unable to control the people (Ex 24:14; 32:1-8), Jesus found that those he had left behind could not cast out a particularly troublesome demon. Jesus casts out the demon immediately, demonstrating how it should be done (17:18; compare 8:26), but the disciples' inability twice invites Jesus' reproof of their weak faith (17:17, 20).
Jesus Honors One Person's Faith on Behalf of Another (17:14-15)
A man brings his son to the One with power to deliver him. Some of the symptoms depicted here resemble those of epilepsy (for example, Alexander 1980:83), which may imply that demons gaining control over the human central nervous system can sometimes cause epileptic-type phenomena. This observation does not, however, mean that epilepsy is always caused by demons; both the differentiation of the two in 4:24 and the numbers of committed Christians who suffer from epilepsy invite us to distinguish the two. Some contemporary accounts of spirit possession tell of spirits seeking to make people burn themselves (Kaplan and Johnson 1964:211).
Jesus accepts the father's faith on behalf of his son. Those who support infant baptism have found in this text a principle they believe supports it (Richardson 1958:359-60); those who emphasize the importance of personal faith at baptism are not persuaded by the analogy. But in either case the principle applies for many other kinds of prayer (compare, for example, 10:8; 18:15-20; 1 Jn 5:16) and encourages us in our faith for others' needs (compare 8:13; 9:2; 15:28).
Jesus Summons Us to Grow in Active Faith (17:16-18)
Jesus expected his disciples to have sufficient faith to repeat his miracles by this point (vv. 16-17, 20). Unbelieving . . . generation (v. 17) applied generally to Jesus' contemporaries (11:16; 12:39-45; 13:39, 45; 16:4; 23:36; 24:34), but in this case specifically to his disciples, who proved unable to stand in for him in his absence (17:16). Disciples were by definition apprentices in training to assume the role of their teachers. Jesus had already sent his disciples out, and they had healed the sick and driven out demons (10:8). Had they not seen enough to believe (compare 8:26)?
Matthew expected his audience to learn from these recorded signs of Jesus, just as the first disciples did when they witnessed them. We who read these accounts in the Bible should be growing in our faith relationship with Jesus, as the disciples did who first walked with him. How often do needs around us go unmet because we neglect radical trust in God, especially on behalf of others' needs?
The Disciples Lacked the Most Basic Level of Faith (17:19-20)
Jesus explicitly attributes their inability to the smallness of their faith (compare 6:30; 8:26; 14:21; 16:8), pointing out that even a mustard seed's worth of faith would be sufficient to cast out not merely demons but mountains (17:20; 21:21; 1 Cor 13:2). The disciples already recognized how small a mustard seed was (Mt 13:32). Ancient peoples thought of mountains as rooted far beneath the earth (Gundry 1982:353), so "moving mountains" was a typical Jewish teacher's image for doing what was virtually impossible. With this illustration Jesus indicates that even were we casting out mountains rather than demons, we would only be scratching the surface of a life of faith. What could we do with faith greater than that of a tiny mustard seed! Like children who have only begun to walk, most of us have only begun our adventure of faith.
Faithful Obedience to God Invites Martyrdom As Well As God's Power (17:22-23)
We may become too infatuated with God's power and protection (v. 20); God sometimes calls us into danger. God twice honored Elijah's call for fire from heaven (2 Kings 1:10-12), but then instructed him to accompany the third captain (who by this point, at least, feared God enough to provide the prophet safe passage). Jesus' disciples had preferred the glories of the messianic kingdom to suffering (Mt 16:16, 21-22; 17:4); like them, we must avoid missing the point of his triumphant empowerment (compare 1 Cor 13:2; Lk 10:17-20). Faith means willingness to go where God leads, not power to avert all unpleasant circumstances. We mature as the Lord leads us through hard tests for his name's sake, forcing us to actively trust his provision and power.
Jesus gives us access to tremendous power for accomplishing his will. Jesus' own example shows us, however, that those who have an intimate faith relationship with God act in compassion for others' needs rather than exploiting power frivolously (Mt 4:3-10).