Then the disciples coming. The disciples wonder that the power which they once possessed has been taken from them; but they had lost it by their own fault. Christ therefore attributes this want of ability to their unbelief, and repeats and illustrates more largely the statement which he had previously made, that nothing is impossible to faith It is a hyperbolical mode of expression, no doubt, when he declares that faith removes trees and mountains; but the meaning amounts to this, that God will never forsake us, if we keep the door open for receiving his grace. He does not mean that God will give us every thing that we may mention, or that may strike our minds at random. On the contrary, as nothing is more at variance with faith than the foolish and irregular desires of our flesh, it follows that those in whom faith reigns do not desire every thing without discrimination, but only that which the Lord promises to give. Let us therefore maintain such moderation as to desire nothing beyond what he has promised to us, and to confine our prayers within that rule which he has laid down. But it may be objected, that the disciples did not know whether or not the Lord was pleased to cure the lunatic It is easy to reply, that it was their own fault if they did not know; for Christ is now speaking expressly about special faith, which had its secret instincts, as the circumstances of the case required. And this is the faith of which Paul speaks, (1-Corinthians 12:9.) How then came it that the apostles were deprived of the power of the Spirit, which they had formerly exercised in working miracles, but because they had quenched it by their indolence? But what Christ said about special faith, in reference to this particular event, may be extended to the common faith of the whole Church.

Then came the disciples - This inquiry was made in some house to which they retired near the place where the miracle was performed (Mark). Jesus told them, in reply, that it was because of their unbelief that they had not been able to cast him out. They were appalled by the difficulty of the case and the obstinacy of the disease. Their faith would not have made it more easy for God to work this miracle, but such was his will - such the way in which he worked miracles, that he required faith in those who were the instruments.

Why could not we cast him out? - They were confounded at their want of success - but not at their want of faith, which was the cause of their miscarriage! When the ministers of the Gospel find their endeavors, with respect to some places or persons, ineffectual, they should come, by private prayer, to Christ, humble themselves before him, and beg to be informed whether some evil in themselves have not been the cause of the unfruitfulness of their labors.

(3) Then came the disciples to Jesus apart, and said, Why could not we cast him out?
(3) Incredulity and distrust hinder and break the direction of God's benefits.

Then came the disciples to Jesus apart,.... Or "secretly", as the Vulgate Latin, and Munster's Hebrew Gospel read; that is, privately, and when alone; and as Mark says, "when he was come into the house"; and was by himself, then came the nine disciples to him, to converse with him about this matter,
and said unto him, why could not we cast him out? That is, the devil, and so cure the lunatic; the Syriac and Persic versions render it, "why could not we heal him?" The lunatic; which only could be done by casting out the demon: they were concerned, fearing they had lost the power which Christ had bestowed on them, and wanted to know what they had done, which had deprived them of it; and what should be the cause of their late unsuccessful attempt, when they had so frequently triumphed over the unclean spirits, that were subject to them. Though they might have learned from the answer Christ gave to the father of the lunatic, and the general character of the Jewish nations in that answer, the true reason of their own inability; but this they took no notice of, imagining it belonged entirely to others, and not to them.

Why could not we cast him out? The answer is, Lack of faith.
This mountain. Lofty Hermon, in plain sight.
Nothing shall be impossible to you. Upon the condition of perfect faith. Compare Hebrews. chap. 11. Faith in Christ, faith exercised in fasting and prayer, are the conditions of power.