Albert Barnes Commentary Matthew 17:19

"Then came the disciples to Jesus apart, and said, Why could not we cast it out?" — Matthew 17:19 (ASV)

Then came the disciples, etc. This inquiry was made in a house to which they had withdrawn near the place where the miracle was performed (Mark 9:28). Jesus told them, in reply, that it was because of their unbelief that they had not been able to cast him out. They were dismayed by the difficulty of the case and the stubbornness of the disease. Their faith would not have made it easier for God to work this miracle; but such was His will, and such was the way He performed miracles, that He required faith in those who were the instruments.