The Reason For Their Failure (17:19-21).
The disciples learn that their failure was due to the lack of quality in their faith. What was needed was the kind of faith that can only be built up by depth in prayer (Mark 9:29). It was their failure to spend their time in continuing prayer that was at the root of their unbelief (Matthew 14:16-27; Matthew 14:31; Matthew 15:5; Matthew 15:8).
Analysis. .
a Then came the disciples to Jesus apart, and said, “Why could we not cast it out?” (Matthew 17:19).
b And he says to them, “Because of your little faith” (Matthew 17:20 a).
b “For truly I say to you, If you have faith as a grain of mustard seed (Matthew 17:20 b).
a “You will say to this mountain, ‘Remove hence to yonder place', and it will remove, and nothing will be impossible to you” (Matthew 17:20 c).
Note that in ‘a' they ask why they could not cast it out, and in the parallel that with proper faith they will be able to cast anything out. In ‘b' their failure was due to little faith, and in the parallel all that is required is faith the size of a grain of mustard seed.