Mark 9:29

(ubi sup.) Further, our Lord, while teaching the Apostles how the worst devil is to be expelled, gives all of us rules for our life; that is, He would have us know that all the more grievous attacks of evil spirits or of men are to be overcome by fastings and prayers; and again, that the anger of the Lord, when it is kindled for vengeance on our crimes, can be appeased by this remedy alone. But fasting in general is not only abstinence from food, but also from all carnal delights, yea, from all vicious passions. In like manner prayer taken generally, consists not only in the words by which we call upon the Divine mercy, but also in all those things which we do with the devotedness of faith in obedience to our Maker, as the Apostle testifies, when he says, Pray without ceasing. (1 Thess. 5:17)

Or else, the folly which is connected with the softness of the flesh, is healed by fasting; anger and laziness are healed by prayer. Each wound has its own medicine, which must be applied to it; that which is used for the heel will not cure the eye; by fasting, the passions of the body, by prayer, the plagues of the soul, are healed.

(ubi sup.) They feared that perchance they had lost the grace conferred upon them; for they had already received power over unclean spirits. It goes on: And he said unto them, This kind can come forth by nothing but by prayer and fasting.

That is, the whole class of lunatics, or simply, of all persons possessed with devils. Both the man to be cured, and he who cures him, should fast; for a real prayer is offered up, when fasting is joined with prayer, when he who prays is sober and not heavy with food.