John Chrysostom
Catena Aurea · Matthäusevangelium 17
Then came the disciples to Jesus apart, and said, Why could not we cast him out? And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. Howbeit this kind goeth not out but by prayer and fasting.
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.