19. Then the disciples, coming to Jesus apart, said, Why could not we cast it out? 20. And Jesus said to them, On account of your unbelief; for verily I say to you, If you have faith as a grain of mustard-seed, you shall say to this mountain. Remove thou hence, and it shall remove; and nothing shall be impossible to you.21. But this kind  goeth not out but by prayer and fasting.
28. And when he had entered into the house, his disciples asked him apart, Why could not we cast out? 29. And he said to them, This kind  cannot go out in any other way than by prayer and fasting.
5. And the apostles said to the Lord, Increase our faith.6. And the Lord said, If you had faith as a grain of mustard-seed, you might say to this sycamore tree. Be thou rooted up and planted in the sea; and it would obey you.
Matthew 17:19. 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.
21. This kind goeth not out,  By this expression Christ reproved the negligence of certain persons, in order to inform them that it was not an ordinary faith which was required; for otherwise they might have replied that they were not altogether destitute of faith The meaning therefore is, that it is not every kind of faith that will suffice, when we have to enter into a serious conflict with Satan, but that vigorous efforts are indispensably necessary. For the weakness of faith he prescribes prayer as a remedy, to which he adds fasting by way of an auxiliary. "You are effeminate exorcist," said he, "and seem as if you were engaged in a mock-battle got up for amusement;  but you have to deal with a powerful adversary, who will not yield till the battle has been fought out. Your faith must therefore be excited by prayer, and as you are slow and languid in prayer, you must resort to fasting as an assistance."  Hence it is very evident how absurdly the Papists represent fasting to be the specific method of driving away devils, since our Lord refers to it for no other reason than to stimulate the earnestness of prayer. When he says that this kind of devils cannot be cast out in any other way than by prayer and fasting, he means that, when Satan has taken deep root in any one, and has been confirmed by long possession, or when he rages with unbridled fury, the victory is difficult and painful, and therefore the contest must be maintained with all our might.
 "Mais ceste sorte [de diables] ne sort point;" -- "but this kind [of devils] goeth not out."  "Ceste espece [de diables] ne sort point;" -- "this kind [of devils] goeth not out."  "Cest espece [de diablos] ne sort point;" -- "this kind [of devils] goeth not out."  "Vous y venez ainsi qu'a un combat de petits enfans, et comme s'il n'estoit question que de s'escarmoucher pour passe-temps." -- "You come to it as if it were to a fight of little children, and as if you had nothing to do but to skirmish for amusement."  "Comme une aide pour vous exciter et enflamber;" -- "as an assistance to excite and inflame you."
 "Ceste espece [de diables] ne sort point;" -- "this kind [of devils] goeth not out."
 "Cest espece [de diablos] ne sort point;" -- "this kind [of devils] goeth not out."
 "Vous y venez ainsi qu'a un combat de petits enfans, et comme s'il n'estoit question que de s'escarmoucher pour passe-temps." -- "You come to it as if it were to a fight of little children, and as if you had nothing to do but to skirmish for amusement."
 "Comme une aide pour vous exciter et enflamber;" -- "as an assistance to excite and inflame you."