Here Christ is not speaking of that faith which believes in Him undoubtingly and knows Him to be true God, but of the faith (needed) to work miracles. If ye have faith, He said, so exceedingly warm and burning as a grain of mustard seed (for these are its qualities), and if it is believed without a doubt that ye will perform signs, then ye will receive such power, that if ye desire to move the very mountains, ye will move them.
-St. John Chrysostom, The Gospel Commentary, edited by Hieromonk German Ciuba, 2002, 4th Century


At this point he is referring to the faith required for signs, and mentions a mustard seed is small in size, yet it is more potent than all others in its effects. So to bring out that the least degree of genuine faith is capable of great things, he mentioned the mustard seed.
-St. John Chrysostom, Homily 56 (PG 58, 549-50) Translated by Robert Charles Hill in "Spiritual Gems from the Gospel of Matthew", 4th Century