Matthew 17:7-21 It would help your understanding if you begin reading at verse one.
What do you suppose the disciples thought when Jesus “raised them up” and they now saw no one but him any longer?
John the Baptist came “in the spirit of Elijah” but (somewhat like Christ for whom he was a forerunner) he was rejected.
If folks today were honest I fear that many would have to admit what this man admits to Jesus in verse 15! Notice Jesus’ response (v. 17). Have you ever felt like this?
Note carefully Jesus’ response to the disciples as to why they couldn’t heal the boy: 1) the littleness of your faith, 2) if you would exercise even the tiniest amount of faith NOTHING would be impossible to you, 3) it takes a life of trusting God, not just an hour a week (v. 21 prayer and fasting practiced often).
For those who want more on why verse 21 is missing in your Bible or why the footnotes to it, ask and I’ll post.