William Burkitt on Matthew 21:20

The disciples being filled with admiration at the sudden withering of the fig-tree, thereupon our Saviour exhorts them to have faith in God; that is, firmly to rely upon the power of God whereby he is able, upon the goodness of God whereby he is willing, to perform his promises to us.

Learn, 1. That faith is a necessary ingredient in prayer. Praying without faith, is like shooting without a bullet; it makes a noise, but doth no execution.

2. That whatsoever good thing God has made the matter of a promise, shall be given to good men, praying in faith. Whatsoever ye ask in prayer, believing, ye shall receive.

Yet, Note, That the faith here promised to root up mountains, must be restrained to that age of miracles, and to the persons to whom this was spoken, namely, the apostles and first propagators of the gospel; it being certain from experience, this is no ordinary and perpetual gift of Christians.