(29) But by prayer and fasting.--The better MSS. omit the last two words. It is possible that they may have been added, like the "tears" of Mark 9:24, to strengthen the words actually spoken, by bringing in what had been found to bring with it a new intensity of spiritual volition, and therefore of power to rescue human souls from the frenzy and despair into which they had been plunged by the unclean spirits that possessed them. A like addition of "fasting" to prayer, apparently from a like ascetic tendency, is found in 1Corinthians 7:5, where see Note. In St. Matthew both words are found, but some of the most ancient MSS. omit the whole verse. On the whole, however, there is a balance of evidence in their favour; and, as shown in the Note on Matthew 17:21, what they teach is in harmony with other portions of the teaching both of our Lord and His Apostles.

9:14-29 The father of the suffering youth reflected on the want of power in the disciples; but Christ will have him reckon the disappointment to the want of faith. Very much is promised to our believing. If thou canst believe, it is possible that thy hard heart may be softened, thy spiritual diseases may be cured; and, weak as thou art, thou mayest be able to hold out to the end. Those that complain of unbelief, must look up to Christ for grace to help them against it, and his grace will be sufficient for them. Whom Christ cures, he cures effectually. But Satan is unwilling to be driven from those that have been long his slaves, and, when he cannot deceive or destroy the sinner, he will cause him all the terror that he can. The disciples must not think to do their work always with the same ease; some services call for more than ordinary pains.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;

this kind can come forth by nothing, but by prayer and fasting; which they had not observed; See Gill on Matthew 17:21.