(19) Why could not we cast him out?--The question came obviously from the disciples who had been left below when our Lord went apart with Peter, James, and John, to the Mount of the Transfiguration. They did not even now see the reason of their failure. They had dealt with this case as they had dealt with others. Why had they not met with a like issue? They did not as yet perceive that they came under our Lord's language of rebuke, and did not look on themselves as belonging to the "faithless generation."

Verse 19. - Apart (κατ ἰδίαν). Jesus had retired to a house (Mark) when the disciples came to him. The question which they desired to ask was one that could not he investigated in the presence of the sneering, unbelieving crowd. Why could not we (ἡμεῖς, emphatic) cast him (αὐρὸ, it) out? They had keenly felt their impotence and failure, so publicly and distressingly displayed, especially as they had received power to eject demons, and had successfully exercised this authority (Luke 10:17). The Lord's rebuke (ver. 17) had passed over their heads, and not been understood as applicable to themselves. So it was with some bitterness that they asked the question. The nine had not been permitted to witness the Transfiguration; they were not even to be made acquainted with this wondrous transaction at present. More preparation, greater receptivity, was required, before they were fit to be admitted to the full mysteries of the kingdom. They had still much to learn, were still only pupils, and their late failure was permitted in order to help them to attain to self-knowledge and more entire self-surrender.

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!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.