Mat 17:20 And Jesus said to them, Because of your unbelief. For truly I say to you, If you have faith like a grain of mustard seed, you shall say to this mountain, Move from here to there. And it shall move. And nothing shall be impossible to you.
Mat 17:20 And Jesus said unto them, because of your unbelief,…. The Arabic and Ethiopic versions read, “because of your little faith”, or “the smallness of your faith”; and so does one Greek manuscript; and which is what is doubtless meant by their unbelief; for they were not altogether destitute of faith, but their faith was very low, and their unbelief very great. Christ says, not because of the unbelief of the parent of the child, and those that were with him, though that also was a reason; but because of their unbelief, being willing to convince them of their unbelief, as he had done the father of the child, who had confessed it, and desired it might be removed from him: but lest they should think they had lost their power of doing miracles, Christ adds;
for verily I say unto you, if ye have faith as a grain of mustard seed; which was a very small seed, the least of all seeds, and is used very often proverbially by the Jews, to signify anything of a small quantity or weight (b), and is sometimes used of faith, as here; so speaking of the congregation of Edom, meaning the Christians, they (c) say,
“they have not אמונה כמו גרעין של חרדל, “faith as a grain of mustard seed”.”
And it is used in like sense in other eastern nations; and by Mahomet in his Alcoran (d), who says,
“We will appoint just balances in the day of resurrection, neither shall any soul be injured at all, although the merit or guilt of an action be of the weight of “a grain of mustard seed”.”
So that it has no reference to the quality of mustard seed, being hot and acrimonious; which has led some interpreters wrong, to compare faith unto it, for its liveliness and fervency: when our Lord only means, that if his apostles had ever so small a degree of faith in exercise, which might be compared for its smallness to this least of seeds, such an effect as he after mentions would follow; and which therefore is to be understood, not of an historical faith, by which men assent to all that is in the Bible as true; nor of a special, spiritual faith, by which souls believe in Christ, as their Saviour and Redeemer; for of neither of these can the following things in common be said; but of a faith of miracles, peculiar to certain persons in those early times, for certain reasons; which such as had but ever so small a degree of, as the apostles here spoken to might say, as Christ observes to them,
ye shall say to this mountain; pointing perhaps to that he was just come down from, which might be in sight of the house where he was,
remove hence to yonder place, and it shall remove: meaning, not that it would be ordinarily or ever done in a literal sense by the apostles, that they should remove mountains; but that they should be able to do things equally difficult, and as seemingly impossible, if they had but faith, when the glory of God, and the good of men, required it. So that it does not follow, because the apostles did not do it in a literal sense, therefore they could not, as the Jew insultingly says (e); since it was meant that they should, and besides, have done, things equally as great as this, and which is the sense of the words. So the apostle expresses the faith of miracles, by “removing mountains”, 1Co_13:2 i.e. by doing things which are difficult, seem impossible to be done: wherefore Christ adds,
and nothing shall be impossible to you; you shall not only be able to perform such a wonderful action as this, were it necessary, but any, and everything else, that will make for the glory of God, the enlargement of my kingdom and interest, the confirmation of truth, and the good of mankind.
(b) T. Bab. Beracot, fol. 3l. 1. Megilla, fol. 28. 2. Nidda, fol. 66. 1. Maimon. lssure Biah, c. 11. sect. 4. Maacolot Asurot, c. 2. sect. 21. &c. 14. sect. 8. Tumaot Okelim, c. 4. sect. 2. & 7. 6. (c) Vet. Nizzachon, p. 148. (d) C. 21. p. 268. & c. 31. p. 336. Ed. Sale. (e) Vet. Nizzachon, p. 237.
Resource: Gill Commentary