Yet if his unbelief was the cause ... why does He blame the disciples? Signifying, that even without persons to bring the sick in faith, they might in many instances work a cure.
St. John Chrysostom
Homily 57 on Matthew 17, 4th Century
For as the faith of the person presenting oftentimes availed for receiving the cure, even from inferior ministers; so the power of the doers oftentimes sufficed, even without belief in those who came to work the miracle.
St. John Chrysostom
Homily 57 on Matthew 17, 4th Century
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
For a man to have such faith appears simple, but it is, on the contrary, something very lofty, not easily attained by many. Such faith is born of boldness before God; but such boldness comes (only) from pleasing God. Beloved, great labour is needed to acquire, through pleasing God, such boldness before Him that one firmly believes that he will grant all that one asks; as it is written, Ask, and it shall be given to you.
St. John Chrysostom
The Gospel Commentary: edited by Hieromonk German Ciuba, 2002., 4th Century
[The Lord] went on to mention mountains as well, and went even further: "Nothing will be impossible for you." Now, for your part, admire [the disciples'] sound values in not concealing their limitations, and the Spirit's power in gradually bringing on those without even a mustard seed in such a way that rivers and fountains of faith gush forth in them.
St. John Chrysostom
Homily 57 (PG 58, 563) Translated by Robert Charles Hill found in "Spiritual Gems from the Gospel of Matthew" , 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