The Apocalypse never directly quotes a passage from the Old Covenant. However, according to several scholars the book’s 404 verses contain from nearly 300 to nearly 600 allusions and echoes of Old Covenant passages. For example, we are told in Revelation 1:1 that God revealed a secret that would shortly come to pass to Jesus who in turn gave it to his angel who then gave it to Jesus’ disciple, John to disclose to the Church. Under the Old Covenant, we are told that it is God who reveals secrets that would come to pass (Daniel 2:28-29), but the Lord wouldn’t do anything before he revealed his secret to his servants, the prophets (Amos 3:7). So, in the very first verse of the Apocalypse we have an allusion to at least two Old Covenant passages.
The Prophets figure prominently in John’s work, but allusions or echoes from all three divisions of the Old Covenant, the Law, the Writings and the Prophets, are found in the Apocalypse. Thus, the question must be asked, if one doesn’t know or understand the Old Covenant properly, is it even possible for that one to understand the Apocalypse?
Paul brings out an interesting point in his second letter to the Corinthians. He mentions the fading glory of the Old Covenant (2Corinthians 3:13-18), as pictured in the veiled face of Moses, which he covered so that Israel wouldn’t see the glory fade away. Paul used this picture to show that the same veil is over the Old Covenant scriptures for the Jews who don’t read them through the eyes of Jesus. The point is that, if the Apocalypse is so saturated with allusions and echoes to the scriptures in the Old Covenant, the reader, if he takes the understanding similar to that of the Jews on those passages, would logically have the same veil over his eyes that is upon that of the Jews today (viz. prophecy concerning judgment, resurrection, the coming of the Messiah etc.).
In the present context of studying the Apocalypse, whose themes cover the coming of the Messiah, the judgment at the end of the age and the resurrection of the dead, the Christian who holds similar beliefs as do the Jews on these subjects, would, in the words of Paul:
“…their minds were hardened; for until this very day at the reading of the Old Covenant (and the New Covenant which interprets the Old Covenant) the same veil remains unlifted, because it is removed in Christ [2Corinthians 3:14 – parenthesis mine].
Understanding when the Apocalypse was written is extremely important to understanding its message. For example, if the book was written during the reign of Domitian, Emperor of Rome, near the end of the first century AD, then it couldn’t possibly be about the fall of Jerusalem in 70 AD. The fact is that many, probably most Biblical scholars, date the Apocalypse late in the first century AD, during the reign of Domitian. These believers spiritualize away the eminency of the internal evidence of a near fulfillment and conclude the book must refer to the distant future. Consequently, the eschatology of these believers is such that they look for a literal, physical return of Jesus, a literal, biological resurrection of the dead, and a final judgment at what they term “the end of time.” Interestingly, first century Jews also looked for a physical Messiah to reign in physical Jerusalem and awaited a physical, bodily resurrection of the dead out of literal graves. If Paul claimed a veil was over the eyes of these first century Jews who rejected Jesus, when they read the scriptures (2Corinthians 3:14), wouldn’t Paul’s claim also include Christians who held similar views that those first century AD Jews embraced about eschatology?
On the other hand, there are other scholars who believe the Apocalypse was written prior to the 70 AD destruction of Jerusalem by the Roman armies. If this is the case, (i.e. that the book was written early), then the internal evidence of a near fulfillment of the Apocalypse, including the coming of Jesus, the judgment and the resurrection must have occurred at or near the destruction of Jerusalem and its Temple. Therefore, knowing when the Apocalypse was written is extremely important to understanding its content. An error in dating this book would necessitate a complete misunderstanding of John’s message. I’ll speak more about this in my next study.
 The actual figure depends upon the individual scholar’s study.