John Chrysostom — as excerpted in the Catena Aurea on Matthew 17:19-21

“The disciples had received from the Lord the power over unclean spirits, and when they could not heal the dæmoniac thus brought to them, they seem to have had misgivings lest they had forfeited the grace once given to them; hence their question. And they ask it apart, not out of shame, but because of the unspeakable matter of which they were to ask. Jesus said unto them, Because of your unbelief.”