mark 9:29 And he said unto them, This kind can come forth by nothing, but by prayer and fasting. - Free Bible Online

      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 Mt 17:20, 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" (Mt 17:20). See on Mr 11:23. "Howbeit this kind goeth not out but by prayer and fasting" (Mt 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 (Lu 9:43) 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.

JFB.