“And when he was come into the house, his disciples asked him privately, Why could not we cast him out?”
King James Version (KJV)
Mr 9:28 Why could not we cast him out? His answer first was, according to Matthew, "Because of your belief" (Mt 17:20). The whole difficulty centered in their want of faith. No demon could defy them if only they had faith enough. And then Jesus illustrates the power of faith by showing that even a small amount of active, living faith, like a grain of mustard-seed, could remove "this mountain", perhaps pointing to Mount Hermon, at whose base they were, and which towered in its grandeur above them.