Matthew 17:21

If then we shall ever be required to be employed in the healing of those who are suffering any thing of this sort, we shall not adjure them, nor ask them questions, nor even speak, as though the unclean spirit could hear us, but by our fasting and our prayers drive away the evil spirits.

"Howbeit, this kind goeth not out, but by prayer and fasting;" meaning the whole kind of evil spirits, not that of lunatics only. Seest thou how He now proceeds to lay beforehand in them the foundation of His doctrine about fasting? Nay, argue not with me from rare cases, that some even without fasting have cast them out. For although one might say this, in one or two instances, of them that rebuke the evil spirits, yet for the patient it is a thing impossible, living luxuriously, to be delivered from such madness: this thing being especially necessary for him that is diseased in that way. "And yet, if faith be requisite," one may say, "what need of fasting?" Because, together with our faith, that also brings no small power. For it both implants much strictness, and of a man makes one an angel, and fights against the incorporeal powers: yet not by itself, but prayer too is needed, and prayer must come first. See, at any rate, how many blessings spring from them both. For he that is praying as he ought, and fasting, hath not many wants, and he that hath not many wants, cannot be covetous; he that is not covetous, will be also more disposed for almsgiving. He that fasts is light, and winged, and prays with wakefulness, and quenches his wicked lusts, and propitiates God, and humbles his soul when lifted up. Therefore even the apostles were almost always fasting. He that prays with fasting hath his wings double, and lighter than the very winds. For neither doth he gape, nor stretch himself, nor grow torpid in prayer, as is the case with most men, but is more vehement than fire, and rises above the earth. Wherefore also such a one is most especially a hater and an enemy to the evil spirits. For nothing is mightier than a man who prays sincerely. For if a woman had power to prevail with a savage ruler, one neither fearing God, nor regarding man; much more will he prevail with God, who is continually waiting upon Him, and controlling the belly, and casting out luxury. But if thy body be too weak to fast continually, still it is not too weak for prayer, nor without vigor for contempt of the belly. For although thou canst not fast, yet canst thou avoid luxurious living; and even this is no little thing, nor far removed from fasting, but even this is enough to pluck down the devil's madness. For indeed nothing is so welcome to that evil spirit, as luxury and drunkenness; since it is both fountain and parent of all our evils.

Or, fasting is here understood generally as abstinence not from food only, but from all carnal allurements, and sinful passions. In like manner prayer is to be understood in general as consisting in pious and good acts, concerning which the Apostle speaks, Pray without ceasing. (1 Thess. 5:17.)

But while He teaches the Apostles how the dæmon ought to be cast out, He instructs all in regulation of life; that we may all know that all the heavier inflictions, whether of unclean spirits, or temptations of men, may be removed by fasts and prayers; and that the wrath also of the Lord may be appeased by this remedy alone; whence he adds, Howbeit this kind is not cast out but by prayer and fasting.

(ord.) Or; This class of dæmons, that is the variety of carnal pleasures, is not overcome unless the spirit be strengthened by prayer, and the flesh enfeebled by fast.