(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.

And when he was come into the house,.... Perhaps into the man's house whose child he had healed, or into some other in these parts, for the sake both of retirement and refreshment:
his disciples asked him privately, why could not we cast him out? The nine disciples, who were particularly concerned in this matter, fearing they had lost the power of casting out devils, conferred upon them, inquired of Jesus, when he was alone, why they could not cast the demon out of the child, when they had ejected unclean spirits out of many others; See Gill on Matthew 17:19.

Why could not we cast him out?

Why could we not cast him out? His answer first was, according to Matthew (Matthew 17:20), "Because of your belief." 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.
Can come forth by nothing, but by prayer. (Fasting is omitted in the New Version, but placed in the margin.) The faith which will be effectual must be a faith exercised in prayer. The work of the Church is still to cast out devils, the unclean spirits of worldliness, selfishness, greed, infidelity, lust, intemperance, Sabbath-breaking. These still defy the ordinary efforts and faith of God's people. We have faith enough for ordinary duties, for some giving, for prayer-meetings, for building churches; but there is a work which ordinary faith cannot do. How shall we get that higher faith? By prayer and fasting. By such an earnestness that we forget the needs of the body.