Matthew 17:21

This kind goeth not out but by prayer, etc. - Τουτο το γενος, this kind, some apply to the faith which should be exercised on the occasion, which goeth not out, doth not exert itself, but by prayer and fasting; but this interpretation is, in my opinion, far from solid. However, there is great difficulty in the text. The whole verse is wanting in the famous Vatican MS., one of the most ancient and most authentic perhaps in the world; and in another one of Colbert's, written in the 11th or 12th century. It is wanting also in the Coptic, Ethiopic, Syriac, Hieros., and in one copy of the Itala. But all the MSS. acknowledge it in the parallel place, Mark 9:29, only the Vatican MS. leaves out νηϚεια, fasting. I strongly suspect it to be an interpolation; but, if it be, it is very ancient, as Origen, Chrysostom, and others of the primitive fathers, acknowledged it. But while candour obliges me to acknowledge that I cannot account for the fact here alleged, that a certain class or genus of demons cannot be expelled but by prayer and fasting, while others may be ejected without them, I can give a sense to the passage which all my readers will easily understand: viz. that there are certain evil propensities, in some persons, which pampering the flesh tends to nourish and strengthen; and that self-denial and fasting, accompanied by prayer to God, are the most likely means, not only to mortify such propensities, but also to destroy them. For other remarkable circumstances relative to this case, see the notes on Mark 9:17, etc.

Howbeit, this kind ... - This kind means this kind of devils - this species of possession. Where they have had long possession where they produce such painful, fixed, and alarming effects, they can be expelled only in connection with prayer and fasting.

Goeth not out but by prayer and fasting - That is, in order to work miracles of this kind to cast out devils in cases so obstinate and dreadful as this, faith of the highest kind is necessary. That faith is produced and kept vigorous only by much prayer, and by such abstinence from food as fits the mind for the highest exercises of religion, and leaves it free to hold communion with God.

17:21 This kind of devils - goeth not out but by prayer and fasting - What a testimony is here of the efficacy of fasting, when addedto fervent prayer! Some kinds of devils the apostles had cast out before this, without fasting.