The disciples had failed in public, and then watched Jesus do with a word what they could not do (Mark 9:18, 25-27). They wanted to know why. They had reason to expect success, because Jesus had earlier given the twelve "power over unclean spirits" (Mark 6:7), and "they cast out many devils" (Mark 6:13).

Gill says the disciples were "fearing they had lost the power of casting out devils" and asked why they could not cast this one out when they had cast unclean spirits out of many others. Matthew Henry says they asked privately so "that wherein they were defective might be made up another time, and they might not again be thus publicly shamed." Their question is honest. Instead of hiding their failure, they brought it to the One who could explain it.

Jesus answered plainly: "This kind can come forth by nothing, but by prayer and fasting" (Mark 9:29). Matthew records that He also said, "Because of your unbelief," and spoke of faith "as a grain of mustard seed" (Matthew 17:20). Then they left and passed through Galilee, and Jesus kept teaching them that He would be killed and rise the third day (Mark 9:30-31).

The Greek texts differ on Jesus' answer. The Textus Receptus, which the KJV follows, reads "by prayer and fasting" (en proseuche kai nesteia). Other Greek editions read only "by prayer," which is why the ESV says, "This kind cannot be driven out by anything but prayer."

On the parallel saying in Matthew 17:21, Barnes explains "this kind" as "this species of possession," one that had held its victim long and fiercely. For such a case, he says, "faith of the highest kind is necessary," and that faith is kept strong by much prayer and by abstaining from food.

This question shows that past success was not enough. The disciples had cast out spirits before (Mark 6:13), but that did not carry them through this case. Jesus' answer turned them back to dependence on God in prayer.

Matthew Henry's Concise Commentary draws the lesson: "The disciples must not think to do their work always with the same ease; some services call for more than ordinary pains." Jesus did not shame them for asking. He answered, and then kept teaching them on the road (Mark 9:30-31).