Albert Barnes Commentary Matthew 17:21

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

Howbeit this kind, etc. This kind means this kind of devils; this species of possession. Where they have had long possession; where they produce such painful, and 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.