Charles Ellicott Commentary Matthew 17:21

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

This kind goeth not out but by prayer and fasting. These words imply that there are degrees in the intensity of evil ascribed to demons, amounting to a difference in kind. Some might yield to the energy of a human will, the power of the divine name, and the prayers of even a weak faith. Others, like the one before us here, required a greater intensity of the spiritual life, gained through the prayer and fasting of which our Lord speaks.

The circumstances of the case make it probable that our Lord himself had graciously fulfilled both conditions. We know the disciples did not yet fast (Matthew 9:14–15), and the facts imply that they had been weak and remiss in prayer.

These words are noteworthy because they testify to the true foundation and motive for fasting, and to the gain for the higher spiritual life obtained by this act of conquest over the lower nature when accompanied by true prayer. Thus, Saint Peter’s vision (Acts 10:9–10) and the appointment of Paul and Barnabas by the direct guidance of the Spirit (Acts 13:2) are both connected with fasting. And Saint Paul, besides the hunger and thirst that came upon him as incidents of his missionary work, speaks of himself as being in fastings often (2 Corinthians 11:27).