Mark 9:29

29. And he said unto them, This kind can come forth by nothing but by prayer and fasting—that is, as nearly all good interpreters are agreed, "this kind of evil spirits cannot be expelled," or "so desperate a case of demoniacal possession cannot be cured, but by prayer and fasting." But since the Lord Himself says that His disciples could not fast while He was with them, perhaps this was designed, as ALFORD hints, for their after-guidance—unless we take it as but a definite way of expressing the general truth, that great and difficult duties require special preparation and self-denial. But the answer to their question, as given in Matthew 17:20; Matthew 17:21 is fuller: "And Jesus said unto them, Because of your unbelief. For verily I say unto you, 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" (Matthew 17:21- :). See on Mark 9:1. "Howbeit this kind goeth not out but by prayer and fasting" (Matthew 17:21), that is, though nothing is impossible to faith, yet such a height of faith as is requisite for such triumphs is not to be reached either in a moment or without effort—either with God in prayer or with ourselves in self-denying exercises. Luke (Matthew 17:21- :) adds, "And they were all amazed at the mighty power of God"—"at the majesty" or "mightiness of God," in this last miracle, in the Transfiguration, c. or, at the divine grandeur of Christ rising upon them daily.

Second Explicit Announcement of His Approaching Death and Resurrection (Matthew 17:21- :).