Jesus’ disappointment with His disciples’ inability to cast the demon out of the epileptic boy is readily felt in the words, “O faithless and perverse generation, how long shall I be with you?” (Matt. 17.17).
Later, in privacy the disciples asked Jesus, “Why could we not cast it out?” (v. 19). When Christ sent the disciples out (Matt. 10.6–8), He explicitly commissioned them to do these kinds of miracles. Less than a year later, they failed where they had once succeeded. Christ’s explanation for their failure was that their faith was deficient (v. 20). The deficiency did not consist in a lack of confidence; they were surprised that they could not cast out this demon. The problem probably lay in a failure to make God—rather than their own gifts—the object of their confidence.
True faith, even “faith as a mustard seed” (v. 20), by Christ’s definition, always involves surrender to the will of God. What He was teaching here is nothing like positive-thinking psychology. He was saying that both the source and the object of all genuine faith—even the weak, mustard seed variety—is God. And “with God nothing will be impossible” (Luke 1.37). Here, Christ assumes the qualifying thought that is explicitly added by 1 John 5.14. what we ask for must be “according to His will.”