{6} And when he was come into the house, his disciples asked him privately, Why could not we cast him out?

(6) We have need of faith, and therefore of prayer and fasting, in order to cast Satan out of that which belonged to him.

People's New Testament

9:28 Why could not we cast him out? His answer first was, according to Matthew, Because of your belief (Mt 17:20). The whole difficulty centered in their want of faith. No demon could defy them if only they had faith enough. And then Jesus illustrates the power of faith by showing that even a small amount of active, living faith, like a grain of mustard-seed, could remove this mountain, perhaps pointing to Mount Hermon, at whose base they were, and which towered in its grandeur above them.

Jamieson-Fausset-Brown Bible Commentary

28. Why could not we cast him out?

Matthew Henry's Concise Commentary

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.