Why could not we? The disciples began to apprehend that they had incurred their Master's displeasure, and had thereby lost their power of working miracles. They come therefore secretly to Jesus Christ, to learn why they could not cast out devils. He answered them, that it was their want of faith, which probably failed them on this occasion, on account of the difficulty of the cure, little reflecting that the virtue of the Lord, which worked in them, was superior to every possible evil of both mind and body.
St. Hilary is of opinion, that during the absence of Christ on the mountain, the fervour of the apostles had begun to abate. (Jansenius)
Note that it was not the suffering victim but the demon who had to be directly rebuked. It may be that he indirectly rebuked the boy and the demon went out of him because it was owing to his sins that the demon had oppressed him. .
In saying, “And I brought him to thy disciples, and they could not heal him, "he covertly accuses the Apostles, whereas that a cure is impossible is sometimes the effect not of want of power in those that undertake it, but of want of faith in those that are to be healed.
Not that we must think that He was overcome by weariness of them, and that The meek and gentle broke out into words of wrath, but as a physician who might seethe sick man acting against his injunctions, would say, How long shall Ifrequent your chamber? How long throw away the exercise of my skill, while I prescribe one thing, and you do another? That it is the sin, and not the man with whom He is angry, and that in the person of this one man He convicts the Jews of unbelief, is clear from what He adds, “Bring him to me.”.
He rebuked him, that is, not the sufferer, but the daemon.
Or, His reproof was to the child, because for his sins he had been seized on bythe daemon.
See herein also his folly, in that before the multitude he appeals to Jesus against His disciples. But He clears them from shame, imputing their failure to the patient himself; for many things show that he was weak in faith. But Headdresses His reproof not to the man singly, that He may not trouble him, butto the Jews in general. For many of those present, it is likely, had improper thoughts concerning the disciples, and therefore it follows, “Jesus answered and said, O faithless and perverse generation, how long shall I be with you, how long shall I suffer you? "His “How long shall I be with you?” shewsthat death was desired by Him, and that He longed for His withdrawal.
The lunatic is figuratively one who is hurried into fresh vices every hour, one while is cast into the fire, with which the hearts of the adulterers burn; or again into the waters of pleasures or lusts, which yet have not strength to quench love.
It may be known also, that not now for the first time, but of a long time, the Lord had borne the Jews’ stubbornness, whence He says, “How long shall I suffer you?” because I have now a long while endured your iniquities, and ye are unworthy of My presence.
In which deed He left an example to preachers to attack sins, but to assist men.
. Do you see how the man has shifted the blame for his own lack of faith upon the disciples, saying that they were too weak to heal? The Lord, therefore, is shaming him for accusing the disciples, saying, "O faithless generation," that is, "It is not so much the fault of the weakness of the disciples as it is of your lack of faith, which, being great, has prevailed over the equal measure of their strength." He rebukes not only this man, but everyone who lacks faith, even the bystanders. By saying, "How long shall I be with you?" Christ shows that He longs for the Passion upon the Cross and His departure from them. For He is saying, "How long shall I live among scoffers and unbelievers?" "And Jesus rebuked him" — whom? He who was lunatic. From this it appears that he, too, lacked faith and his lack of faith had given occasion for the demon to enter him.
