John Calvin Commentary Matthew 17:21

"[But this kind goeth not out save by prayer and fasting.]" — Matthew 17:21 (ASV)

This kind goes not out,⁴⁹² By this expression Christ reproved the negligence of certain persons, in order to inform them that it was not an ordinary faith that 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 as an auxiliary. “You are too delicate an exorcist,” He said, “and seem as if you were engaged in a mock battle staged for amusement;⁴⁹³ but you have to deal with a powerful adversary, who will not yield until the battle has been fully fought. Your faith must therefore be stirred up by prayer, and as you are slow and languid in prayer, you must resort to fasting as an aid.”⁴⁹⁴

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 demon cannot be cast out in any other way than by prayer and fasting, He means that when Satan has taken deep root in anyone, 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.

⁴⁹² “Cest espece [de diablos] ne sort point;” — “this kind [of devils] goeth not out.”;” — “this kind [of devils] goeth not out.”

⁴⁹³ “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.”.” — “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.”

⁴⁹⁴ “Comme une aide pour vous exciter et enflamber;” — “as an assistance to excite and inflame you.”;” — “as an assistance to excite and inflame you.”