Origen
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.
Or, all faith is likened to a grain of mustard-seed, because faith is looked on with contempt by men, and shews as something poor and mean; but when a seed of this kind lights upon a good heart as itssoil, it becomes a great tree. The weakness of this lunatic’s faith is yet so great, and Christ is so strong to heal him amidst all his evils, that He likens it to a mountain which cannot be cast out but by the whole faith of him who desires to heal afflictions of this sort.