The majority of contemporary translations exclude these words from the book of Matthew. In Mark’s account, prayer is included, and a later footnote (Mark 9:29) adds a statement about fasting. It’s thought by some that according to Jesus’ comments in Mark, some demons require more work to cast out, but this is simply not true.
Jesus claimed that the main reason the disciples were unable to cast the demon out was due to their lack of faith in who Jesus really is. In the previous passage from Matthew (Matthew 17:17). He did not state that their main problem was a lack of fasting and prayer. It’s more about living a life of prayer and fasting, a consistent life of fellowship with God and not some kind of cause and effect type of limitation. The real mistake—the root of their failure—was that they did not have enough faith in God’s ability to use them. Jesus made it abundantly obvious that if they had faith, nothing would be difficult for them.
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