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.

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.

{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.

17:19 Why could not we cast him out? The answer is, Lack of faith.

17:14-21 The case of afflicted children should be presented to God by faithful and fervent prayer. Christ cured the child. Though the people were perverse, and Christ was provoked, yet care was taken of the child. When all other helps and succours fail, we are welcome to Christ, may trust in him, and in his power and goodness. See here an emblem of Christ's undertaking as our Redeemer. It encourages parents to bring children to Christ, whose souls are under Satan's power; he is able to heal them, and as willing as he is able. Not only bring them to Christ by prayer, but bring them to the word of Christ; to means by which Satan's strong-holds in the soul are beaten down. It is good for us to distrust ourselves and our own strength; but it is displeasing to Christ when we distrust any power derived from him, or granted by him. There was also something in the malady which rendered the cure difficult. The extraordinary power of Satan must not discourage our faith, but quicken us to more earnestness in praying to God for the increase of it. Do we wonder to see Satan's bodily possession of this young man from a child, when we see his spiritual possession of every son of Adam from the fall!