The disciples were amazed at how fast the tree had died. Their words focus on one thing: how soon. Gill says they marvelled "not that Christ should curse it, but that it should wither away so soon." Barnes, comparing Mark, explains that the disciples saw the withered tree "on the morning following that on which it was cursed" (Mark 11:20), and that it was Peter who spoke (Mark 11:21). In Greek their words begin with "How?" so their wonder also carries a question: how could a word do this?

Jesus did not explain how the tree withered. He turned their wonder toward faith: "If ye have faith, and doubt not, ye shall not only do this which is done to the fig tree, but also if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done" (Matthew 21:21). Then He added a promise about prayer: "And all things, whatsoever ye shall ask in prayer, believing, ye shall receive" (Matthew 21:22). In Mark, His answer begins, "Have faith in God" (Mark 11:22).

The KJV prints the disciples' words as an exclamation, but the Greek begins with pos, "how," which is why the ESV renders it as a question: "How did the fig tree wither at once?" The word behind "soon" is parachrema, which Thayer defines as "immediately, forthwith, instantly." It is the same word translated "presently" in Matthew 21:19, where "presently the fig tree withered away."

Mark adds that "the time of figs was not yet" (Mark 11:13). Barnes explains that Jesus acted "according to the appearance of things": the tree was in leaf, and leaves gave reason to look for fruit.

The fig tree stands between the cleansing of the temple (Matthew 21:12-13) and the leaders' challenge to Jesus' authority (Matthew 21:23). Matthew Henry writes that this cursing "represents the state of hypocrites in general," and that Christ "comes to many, seeking fruit, and finds leaves only." Gill goes further and calls the tree "an emblem of the Jews" of that day. Barnes is more cautious: he grants that it would be a good illustration of the destruction of the Jewish people, but says there is not the least evidence that Jesus intended it as such. The rest of the chapter keeps returning to fruit: the householder sends for "the fruits" of his vineyard (Matthew 21:34), and the kingdom is given "to a nation bringing forth the fruits thereof" (Matthew 21:43).

Yet Jesus uses the moment to teach, not only to warn. The same power that withered the tree is the power faith relies on in prayer (Matthew 21:21-22). Barnes suggests the "mountain" probably referred to the difficulties and trials the disciples would face in preaching the gospel.