NOTES for Mat 17:14-21
Having come down from the Mount of Transfiguration, Christ again plunges into a world where the forces of darkness overpower the suffering. But those to whom He gave power to resist darkness do not cope with the ministry entrusted to them. And so the man whose son is tormented by an unclean spirit appeals directly to Christ. Of course, Christ does not leave the request unanswered and drives out the demon. But He did not choose the disciples so that they would retreat before the forces of evil.
Here, immediately after the Transfiguration, we see the contrast between what is manifested by Christ and what the apostles do, or rather cannot do, in His absence. And then we see His anger directed at the disciples.
On the pages of the Gospel we encounter more than once episodes in which Jesus' appearance looks stern. For someone whose idea of Christ was once formed by sugary pictures, the reality of the Gospel may seem inelegant. But Christ's love is not sentimentality; it is real love that takes upon itself pain for the beloved. And often cries out from pain.
Meanwhile, His remark about fasting and prayer forces us to suppose that the disciples tried to act in some other way, perhaps by some "folk" methods, or, simply put, by methods far from genuinely biblical ones. If so, the angry rebuke of the disciples for their unbelief is all the more understandable. (Note that by calling the disciples a faithless and corrupt generation, Christ almost repeated the definition He had earlier given the Pharisees: an evil and adulterous generation. Thus the disciples were likened to the Pharisees.)
But after giving them a "dressing-down," He does not send the disciples away; He continues patiently to teach them.