He replied, This kind can come out only by prayer.
Matthew Henry
The reason he gave to the disciples why they could not cast out this devil. They enquired of him privately why they could not, that wherein they were defective might be made up another time, and they might not again be thus publicly shamed; and he told them, This kind can come forth by nothing but prayer and fasting. Whatever other difference there really might be, none appears between this and other kinds, but that the unclean spirit had had possession of this poor patient from a child, and that strengthened his interest, and confirmed his hold. When vicious habits are rooted by long usage, and begin to plead prescription, like chronical diseases that are hardly cured. Can the Æthiopian change his skin? The disciples must not think to do their work always with a like ease; some services call them to take more than ordinary pains; but Christ can do that with a word's speaking, which they must prevail for the doing of by prayer and fasting.
John Gill
And he said unto them Matthew, besides the following reason, assigns another, as given by our Lord, why they could not cast out the foul spirit, which was their unbelief; of which they were guilty in some sort, as well as the Jews, and the father of the child; but Mark omits it, and only relates this as the reason;
(Mt 17:21) that is, in the exercise of a miraculous faith, expressed in solemn prayer to God, joined with fasting. It seems that Christ not only suggests, that faith was greatly wanting in his disciples; for which reason they could not cast out the devil, and heal the lunatic; but they had been wanting in prayer to God, to assist them in the exercise of their miraculous gifts; and that whilst Christ, and the other three disciples were on the mount, they had been feasting and indulging themselves with the people, and so were in a very undue disposition of mind, for such extraordinary service, for which our Lord tacitly rebukes them. This agrees with the notions of the Jews, who think that, by fasting, a divine soul "may obtain that which is sought for"; and that among other things, for which a private person may afflict himself with fasting, this is one.
David Brown
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. Howbeit this kind goeth not out but by prayer and fasting", 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 (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.
