Here is a good place to examine a statement that has often been cited as proof that the book of Revelation was written long after Jerusalem was destroyed, in the days of the Roman emperor Domitian. It comes from Irenaeus, a bishop in Lyons, France in the latter half of the second century. In his youth he had known Polycarp, an old disciple who had actually seen John the apostle. Irenaeus, in discussing the identity of the man whose number is 666 in Rev. 13:18, says:
"We will not, however, incur the risk of pronouncing positively as to the name of Antichrist; for if it were necessary that his name should be distinctly revealed in this present time, it would have been announced by him who beheld the apocalyptic vision. For that was seen no (sic) very long time since, but almost in our day, towards the end of Domitian's reign."
This is translated from the following Greek paragraph (with apologies for the size of the image):
Almost universally it is assumed that it was the Revelation that was seen toward the end of Domitian's reign (AD81-96). However the phrase "that was seen" is from a masculine verb in the original, and may refer either to John OR the Revelation. The subject matter is John, who would have named the man whose number was 666 if it had been necessary. So, it was JOHN who was seen as late as the last years of Domitian's reign, NOT the Revelation (see John 21:20-25).
This assumption can be traced back to Eusebius in his Ecclesiastical History. In speaking of the persecution under the Roman emperor Domitian, Eusebius states:
"In this persecution, it is handed down by tradition, that the apostle John, who was yet living, in consequence of his testimony to the divine word, was condemned to dwell on the island of Patmos. Irenaeus, indeed, in his fifth book against the heresies, where he speaks of the calculation formed on the epithet of Antichrist, in the abovementioned revelation of John, speaks in the following manner respecting him. 'If, however, it were necessary to proclaim his name, (i.e. Antichrist,) openly at the present time, it would have been declared by him who saw the revelation, for it is not long since it was seen, but almost in our own generation, at the close of Domitian's reign.'" (Ecclesiastical History, pp. 101-102)
Notice that he assumes that the "it" (more properly, "that") of Irenaeus' statement refers to the Revelation, and not to John. His assumption has been followed by thousands for no other reason than that it is a very old assumption!
This has been done to the great detriment of our clear understanding of the Revelation.