"The contrast between the glory of the Transfiguration and Jesus' disciples' tawdry unbelief (see v. 17) is part of the mounting tension that magnifies Jesus' uniqueness as he moves closer to his passion and resurrection."679
17:14-16 The Greek word gonypeteo, translated "falling on his knees"or "knelt,"suggests humility and entreaty, not worship (cf. 27:29; Mark 1:40; 10:17). Likewise "Lord"was simply a respectful address (cf. 8:2). The young man's epilepsy was evidently a result of demon possession (v. 18). The impotent disciples were some or all of the nine who did not go up the mountain for the Transfiguration.
There are many instances of the disciples' failures in this section of Matthew (cf. 14:16-21, 26-27, 28-31; 15:16, 23, 33; 16:5, 22; 17:4, 10-11). Earlier they had great miraculous powers (10:1, 8). However, their power was not their own; it came from Jesus. As Jesus progressively trained the disciples, He also withdrew some of their power to teach them that it came from Him and related to their faith in Him (14:26-17, 31; 15:5, 8).
17:17-18 Jesus' rebuke recalls Moses' words to Israel in Deuteronomy 32:5 and 20. Unbelief characterized the generation of Jews that had rejected Jesus, and now it marked His disciples to a lesser extent. Their failure to believe stemmed from moral failure to recognize the truth rather than from lack of evidence, as the combination of "perverse"and "unbelieving"makes clear (cf. Phil. 2:15). The disciples, too, were slow to believe, slower than they should have been. Jesus' two rhetorical questions expressed frustration and criticism.
17:19-21 The "we"in the disciples' question is in the emphatic position in the Greek text. The problem, as Jesus explained, was their weak faith (Gr. oligopistia). It was not the quantity of their faith that was deficient but its quality. In spite of the revelation of Jesus that they had received, the disciples had not responded to it with trust as they should have done. They had some faith in Jesus, but it should have been stronger.
"Much earlier, Jesus had endowed the disciples with authority to exorcise demons as part of their mission to Israel (10:1, 8). Consequently, he expects them to draw on this authority. But if they approach the tasks of their mission forgetful of their empowerment and encumbered by a crisis of trust, they render themselves ineffectual."680
Removing mountains is a proverbial figure of speech for overcoming great difficulties (cf. Isa. 40:4; 49:11; 54:10; Matt. 21:21-22; Mark 11:23; Luke 17:6; 1 Cor. 13:2). In this context the difficulties in view involved exercising the authority that Jesus had delegated to them to heal people. The disciples were treating the gift of healing that Jesus had given them as a magical ability that worked regardless of their faith in Him. Now they learned that their power depended on proper response to revelation, namely dependent confidence in Jesus to work through them to heal. Continual dependence on Jesus rather than simply belief in who He is constitutes strong faith (cf. Mark 6:5-6).
"Nothing is impossible for the disciple of Jesus who with faith works within the established will of God. It is therefore the case that not every failure in the performance or reception of healing is the result solely of insufficient faith."681
Verse 21 does not occur in several important ancient manuscripts. Evidently copyists assimilated it from Mark 9:29: "And He said to them, This kind cannot come out by anything but prayer.'"
The lesson of this miracle for the disciples was that simple belief that Jesus is the King may be adequate when a person first realizes who Jesus is. It can even result in spectacular miracles. However with the privilege of added revelation about the person and work of Jesus comes increased responsibility to trust totally in Him. Failure to do this weakens faith and restricts Jesus' work through the disciple (cf. John 15:5).
