Believers in preterism, need the Book of Revelation to be written before the destruction of Jerusalem in 70AD.
If John wrote it after 70AD, then it is a fully prophetic book speaking of a future time.
One of the great resources for understanding and reading about church history is the writings of Eusebius.
Here is what he said about John …..
Chapter XVIII.—The Apostle John and the Apocalypse.
1. It is said that in this persecution the apostle and evangelist John, who was still alive, was condemned to dwell on the island of Patmos in consequence of his testimony to the divine word.
2. Irenæus, in the fifth book of his work Against Heresies, where he discusses the number of the name of Antichrist which is given in the so-called Apocalypse of John,
3. “If it were necessary for his name to be proclaimed openly at the present time, it would have been declared by him who saw the revelation. For it was seen not long ago, but almost in our own generation, at the end of the reign of Domitian.”
4. To such a degree, indeed, did the teaching of our faith flourish at that time that even those writers who were far from our religion did not hesitate to mention in their histories the persecution and the martyrdoms which took place during it.
5. And they, indeed, accurately indicated the time. For they recorded that in the fifteenth year of Domitian 7717 was exiled with many others to the island of Pontia in consequence of testimony borne to Christ.
Emperor Domitian reigned from 81AD to 96 AD.