Charles Ellicott 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)

Because of your unbelief — The variant reading, “Because of your little faith,” found in many but not the most authoritative manuscripts, is an interesting example of the tendency to soften the apparent severity of our Lord’s words. This shows conclusively that the disciples themselves fell under the scope of His rebuke to the “faithless and perverse generation.”

If you have faith as a grain of mustard seed — The hyperbolic form of our Lord’s words, repeated later in Matthew 21:21, excluded the possibility of a literal interpretation from the minds of the disciples, as it does from our own. The “grain of mustard seed” was, as in Matthew 13:31, the proverbial symbol of something infinitely small. To “remove mountains” was, as we see in 1 Corinthians 13:2 (which may, however, have been an echo of our Lord’s teaching), the proverbial symbol of overcoming seemingly insurmountable difficulties.

We may believe the words were dramatized by a gesture pointing to the mountain from which our Lord and the three disciples had descended, just as a similar act was later done in reference to the Mount of Olives (Matthew 21:21).

Nothing shall be impossible for you — The words, as absolute as they sound, are nevertheless, by that very fact, conditional. Nothing that falls within the realm of faith in God's wisdom and love—and therefore of submission to His will—is beyond the reach of prayer.