Because of your unbelief. The Revised Version adopts the reading, little faith, in accordance with the best authorities; but it looks like a softening of the original term "unbelief," which corresponds better with Christ's own censure, "faithless generation."
Jesus gives two reasons for the apostles' failure, one connected with their own moral condition, and one (Matthew 17:21) derived from the nature of the demons exorcised. They had, indeed, shown some faith by making even the attempt at the expulsion of the devil, and were not to be classed with the unbelieving scribes; but they had acted in a half-hearted manner, and had not displayed that perfect confidence and trust which alone can win success and make all things possible.
Verily I say unto you. The Lord proceeds to give that lesson concerning perfect faith and its results, which he afterwards repeated in connection with the withered fig tree (Matthew 21:21, where see note) and elsewhere (Luke 17:6).
If ye have faith as a grain of mustard seed, which, as he says (Matthew 13:32), "is less than all seeds." He means a faith real and trustful, though it be small and weak. The phrase is proverbial, expressive of littleness and insignificance.
The mustard seed is quite little, but, grown in favourable soil and under sunny skies, it becomes, as it were, a tree among herbs, so that birds may nestle in its branches. To it faith is compared, because, small at first, it contains within itself power of large development and increase; from minute grains copious results are produced.
Ye shall say unto this mountain. He points to the hill of Hermon, where the Transfiguration had taken place. Remove hence. It is usual to consider the expression here as an Eastern hyperbole, not to be taken literally, but meaning merely that the greatest difficulties may be overcome by faith.
This may be true, but it seems hardly adequate to the explanation of our Lord's emphatic words. St. Paul writes in a similar strain (1 Corinthians 13:2), "If I have all faith, so that I could remove mountains;" where there is nothing necessarily hyperbolical in the supposition.
It seems rather that Jesus meant his words to be received literally, implying that if such a removal as he mentioned was ever expedient and in accordance with God's will, it would be effected by the power of faith; not that he hereby sanctioned an arbitrary and wanton display of miraculous power, but he gives an assurance that, were such a measure rendered necessary for the cause of religion, it would be performable at the call of one whose whole trust was centred on God, and whose will was one with God's will.
Mediaeval writers, followed by later Roman Catholic commentators, give instances of such stupendous effects of faith. The evidence of such miracles is, of course, defective, and would not satisfy modern criticism, but the existence of such legends proves that a literal view was taken of our Lord's saying.
Nothing shall be impossible unto you. The man of faith is practically omnipotent; moral and material difficulties vanish before him.