Matthew 17:19

Of the changefulness of the sinner it is said, The fool changes as the moon. (Ecclus. 27:12.) We may see sometimes that an impulse towards good works comes over such, when, lo! again as by a sudden seizure of a spirit they are laid hold of by their passions, and fall from that good state in which they were supposed to stand. Perhaps his father stands for the Angel to whom was allotted the care of this lunatic, praying the Physician of souls, that He would set free his son, who could not be delivered from his suffering by the simple word of Christ's disciples, because as a deaf person he cannot receive their instruction, and therefore he needs Christ's word, that henceforth he may not act without reason.

"Then came His disciples unto Him apart, and asked Him, why they could not themselves cast out the devil." To me they seem to be in anxiety and fear, lest haply they had lost the grace, with which they had been entrusted. For they received power against unclean spirits. Wherefore also they ask, coming to Him apart; not out of shame (for if the fact had gone abroad, and they were convicted, it were superfluous after that to be ashamed of confessing it in words); but it was a secret and great matter they were about to ask Him of. What then saith Christ? "Because of your unbelief," saith He; "for if ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove, and it shall remove; and nothing shall be impossible unto you." Now if you say, "Where did they remove a mountain?" I would make this answer, that they did far greater things, having raised up innumerable dead. For it is not at all the same thing, to remove a mountain, and to remove death from a body. And certain saints after them, far inferior to them, are said actually to have removed mountains, when necessity called for it. Whereby we see that these also would have done the same, need calling on them. But if there was then no need for it, do not thou find fault. And besides, He Himself said not, "ye shall surely remove it," but "ye shall be able to do even this." And if they did it not, it was not because they were unable (how could this be, when they had power to do the greater things?), but because they would not, there being no need. And it is likely that this too may have been done, and not have been written; for we know that not all the miracles they wrought were written. Then however they were in a state by comparison very imperfect. What then? Had they not at that time so much as this faith? They had not, for neither were they always the same men, since even Peter is now pronounced blessed, now reproved; and the rest also are mocked by Him for folly, when they understood not His saying concerning the leaven. And so it was, that then also the disciples were weak, for they were but imperfectly minded before the cross. But by faith here He means that which related to the miracles, and mentions a mustard seed, to declare its unspeakable power. For though in bulk the mustard seed seem to be small, yet in power it is the strongest of all things. To indicate therefore that even the least degree of genuine faith can do great things, He mentioned the mustard seed; neither by any means did He stop at this only, but added even mountains, and went on beyond that. "For nothing," saith He, "shall be impossible to you." But do thou herein also marvel at their self-denial, and the might of the Spirit; their self-denial in not hiding their fault, and the might of the Spirit in so leading on by degrees them who had not so much as a grain of mustard seed, that rivers and fountains of faith sprang up within them.