Matthew 17:21 - Ellicott's Commentary On The Whole Bible

This kind goeth not out but by prayer and fasting. — The words imply degrees in the intensity of the forms of evil ascribed to demons amounting to a generic difference. Some might yield before the energy of a human will, and the power of the divine Name, and the prayers even of a weak faith. Some, like that which comes before us here, required a greater intensity of the spiritual life, to be gained by the “prayer and fasting” of which our Lord speaks. The circumstances of the case render it probable that our Lord himself had vouchsafed to fulfil both the conditions. The disciples, we know, did not as yet fast (Matthew 9:14-15), and the facts imply that they had been weak and remiss in prayer. The words are noticeable as testifying to the real ground and motive for “fasting,” and to the gain for the higher life to be obtained, when it was accompanied by true prayer, by this act of conquest over the lower nature. So St. 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 St. Paul, besides the “hunger and thirst” that came upon him as the incidents of his mission-work, speaks of himself as “in fastings often” (2 Corinthians 11:27).