The disciples had failed to cure the boy, and then watched Jesus rebuke the devil and cure him (Matthew 17:16-18). They wanted to know what had gone wrong. Mark says they asked Him "privately" once He had come into the house (Mark 9:28).

Gill thinks they were troubled, afraid that they had lost the power Christ had given them, and that they did not yet see that the fault lay with their own unbelief. Barnes says they were appalled by how hard the case was, and doubted whether they could cure him. Their question is honest. Instead of hiding their failure, they brought it to the One who could explain it.

Jesus answered plainly: "Because of your unbelief" (Matthew 17:20). Then He gave a promise: "If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you." In the KJV He adds, "Howbeit this kind goeth not out but by prayer and fasting" (Matthew 17:21). Mark records the same lesson: "This kind can come forth by nothing, but by prayer and fasting" (Mark 9:29).

Greek manuscripts differ on Jesus' answer. The Textus Receptus, the Greek text behind the KJV, reads apistia, unbelief. Other manuscripts read oligopistia, little faith, which is why the ESV says "Because of your little faith." The ESV also leaves out Matthew 17:21, because that verse is missing from many manuscripts, while the same saying stands in Mark 9:29.

The mustard seed was proverbially tiny. Barnes calls it the smallest of all seeds, and says a mountain stood for the hardest tasks, things that at first look impossible.

The disciples' question shows where real power comes from. They had not been doing something new. They had simply been unable to do it. Barnes explains that God did not need their faith to make the miracle easier, but it was His will to require faith in those He used. The power was always Christ's, received by trusting Him.

Matthew Henry draws a careful line: "It is good for us to distrust ourselves and our own strength," but "it is displeasing to Christ when we distrust any power derived from him." Jesus' answer does not shame the disciples. It points them away from themselves and back to faith and prayer.