Within the context of this chapter, it does not seem like John ever prophesies. However, within the time limits of the context, he did prophesy. Prophesy here does not mean "to foretell" but "to speak under inspiration"—of God—to preach!
Notice Revelation 11:1: "Then I was given a reed like a measuring rod. And the angel stood, saying, 'Rise and measure the temple of God, the altar, and those who worship there.'"
Chapters 10 and 11 are one prophecy. The Seven Thunders have something to do with John, who is receiving the revelation. They also have something to do with the Two Witnesses. The Seven Thunders will sound, and they will all finish their sounding before the Two Witnesses and before the seventh seal's seventh trumpet.
When will John preach again? We know that the Two Witnesses will preach after the time of this verse. We can be certain that John himself will not preach. He was almost a hundred years old by the time Christ gave him the book of Revelation, and he was about ready to die. The ones who will "prophesy again" are the Two Witnesses and at a much later time. They will be antitypes of John. They will preach again what has previously been preached.
Who did the prophesying? The Seven Thunders did! They said the things John was not allowed write. What nickname did Jesus give to John and his brother James? "The Sons of Thunder" (Mark 3:17)! What did James and John preach? The gospel of the Kingdom of God! Jesus tells John to "prophesy again," but he will not literally do so any more than John the Baptist was literally Elijah! John the Baptist came in the spirit and power of Elijah, an antitype of Elijah. The Two Witnesses will be antitypes of John and James—"the Sons of Thunder!"
John W. Ritenbaugh
Revelation 10 and the Laodicean Church
Other Forerunner Commentary entries containing Revelation 10:11: