Matthew 21:21

Jesus answered and said unto them
His disciples wondering at his power, in causing the fig tree to wither so suddenly:

verily I say unto you, if ye have faith;
that is, in God, in his power, which reaches to all things: the object of faith is expressed in Mark, and by way of exhortation, "have faith in God", that he will enable you to perform whatsoever ye shall desire; which must be understood, not of spiritual faith in the promises of God, and person of Christ, but of, the faith of miracles, or faith in the power of God to perform things that are above the strength of nature:

and doubt not;
either of the power, or will of God to do for you, and by you, the thing desired; for this kind of faith would not admit of the least degree of doubting: there must be no hesitation in the mind, no reasoning upon the thing, how it can be performed; the mind must not be divided between the power and will of God, and the difficulties and discouragements which attend the case, but must believe in hope against hope, with a full persuasion of accomplishment: for want of this faith, without doubting, the disciples could not cure the child that was lunatic.

Ye shall not only do this which is done to the fig tree;
cause one to be dried up, and wither away by a word, as Christ had done to this, which, comparatively speaking, was but a lesser sort of miracle;

but also, if ye shall say to this mountain;
the Mount of Olives, where Christ and his disciples now were, and were passing over, or, at least, were very near it; or any other mountain wherever they might be, to which they should, upon any occasion, think fit to say,

be thou removed, and cast into the sea;
which was many miles off from Mount Olivet, and must he a very surprising performance for a mountain to be rooted up, so large as that was, and be carried several miles from its former situation, and be thrown into the sea; and yet, as difficult and amazing as this may seem,

it shall be done:
that is, provided the person doubts not; or, as it is said in Mark, "shall not doubt in his heart, but shall believe that those things, which he saith, shall come to pass, he shall have whatsoever he saith": for this must not be confined to the particular instances of drying up a fig tree, or removing a mountain, but the doing of any sort of miracle, how great soever. Nor is it our Lord's meaning that they should do these particular things; nor is it certain that they ever did: but his sense is, that, had they faith, they should be able not only to do such lesser miracles, as, comparatively speaking, the withering of the fig tree was, but they should be able to perform things much more difficult and surprising, whenever the good of the souls of men, the propagation of the Gospel, and the glory of God required them.