Matthew 17:20

Because of your unbelief - Are we preachers of the Gospel? Do the things of God rest upon our minds with a deep and steady conviction? Can we expect that a doctrine which we do not, from conviction, credit ourselves, can be instrumental in our hands of begetting faith in others? So we preached, end so ye believed. The word preached generally begets in the people the same spirit which the preacher possesses. Instead of Ϛ , unbelief, the famous Vatican MS. and Cod. Cyprius, six others, Coptic, Ethiopic, Armenian, and Arabic, Origen, and Chrysostom, read Ϛ , littleness of faith. The disciples had some faith, but not enough - they believed, but not fully.

As a grain of mustard seed - Some eminent critics think this a proverbial expression, intimating a Great Degree of faith, because removing mountains, which St. Paul, I Corinthians 13:2, attributes to All Faith; i.e. the greatest possible degree of faith, is attributed here, by our Lord, to that faith which is as a grain of mustard seed. However this may be, there can be no doubt that our Lord means, as Bishop Pearce well remarks, a thriving and increasing faith; which like the grain of mustard seed, from being the least of seeds, becomes the greatest of all herbs; even a tree in whose branches the fowls of the air take shelter. See Wakefield' s Comment, and the note on Matthew 13:32 (note).