How can Revelation be a "sealed book" when it clearly shows that the seals were broken by Christ and then the events within those seals were described from Rev 4 to Rev 18?The answer is Revelation is a sealed book [in large part] just like Daniel was sealed to the Jews.
Daniel was sealed until Revelation was written, but it is no longer sealed. Hence Christ said "let him who reads understand" when speaking of the Abomination of Desolation. That "abomination" is in fact an idol and an image of the Antichrist as noted in Revelation 13. And that image in the future temple will trigger the Great Tribulation.
Also, Christ said to John that "the time is an hand". Yet many assume that the events of the first five seals are in the future. But all one has to do is compare those events with what is described in the Olivet Discourse, and see that those events began immediately after Revelation was completed. For example, false messiahs (Christs) began to appear on the scene almost immediately, and have continued to do so. The Antichrist will be the ultimate false messiah
What is in the future are the events of the 6th and 7th seals.