A.T. Robertson Commentary Matthew 17:20

"And he saith unto them, Because of your little faith: 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." — Matthew 17:20 (ASV)

Little faith (ολιγοπιστιαν). A good translation. It was less than "a grain of mustard seed" (κοκκον σιναπεως). See 13:31 for this phrase. They had no miracle faith. Bruce holds "this mountain" to be the Mount of Transfiguration to which Jesus pointed. Probably so. But it is a parable. Our trouble is always with "this mountain" which confronts our path. Note the form μεταβα (μετα and βηθ).