The date of the composition of St. John's Apocalypse is a critical issue when it comes to the question of how to interpret the many signs and symbols contained in the book. There are two competing opinions on this subject: one school of thought dates the Apocalypse to the close of the first century AD, during the reign of the Roman Emperor Domitian, or approximately AD 95-98; another school of thought places the time of composition before the fall of Jerusalem, or approximately AD 68-70.
Evidence for the later date rests largely on the witness provided by St. Irenaeus, and indeed, his testimony is said by some scholars to be the sole evidence for the later date - thus, for example, Chilton writes, "St. Irenaeus, incidentally, is the only source for this late dating of Revelation; all other 'sources' are simply quoting from him. It is thus rather disingenuous for commentators to claim, as Swete does, that 'Early Christian tradition is almost unanimous in assigning the Apocalypse to the last years of Domitian.'" (D. Chilton, Days of Vengeance [Ft. Worth, TX: Dominion Press, 1987], p. 16)
Given the importance of St. Irenaeus' testimony in this discussion, then, it will be necessary to look at what he wrote, and examine the arguments both for and against this testimony. The particular passage is found in St. Irenaeus' monumental work Against Heresies, but as Gentry notes, "Although originally composed in Greek, today this work exists in its entirety only in Latin translation." (K. Gentry, Before Jerusalem Fell [Tyler, TX: Institute for Christian Economics, 1989], p. 46) However, Gentry continues, "the particular statement in question is preserved for us in the original Greek in Eusebius's Ecclesiastical History." (ibid., p. 46)
The passage is reproduced below in transliterated Greek, followed by the English rendering taken from Schaff's Nicene and Post-Nicene Fathers, Series II:
ei de edei anaphandon en to nun kairo keruttesthai tounoma autou, di ekeinou an errethe tou kai ten apokalupsin heorakotos oude gar pro pollou chronou heorathe, alla schedon epi tes hemeteras geneas, pros to telei tes Dometianou arches.
It is obvious why this statement of St. Irenaeus carries such weight. The saint was not at all far removed from St. John himself, having been acquainted with St. Polycarp, who knew St. John personally. His statement regarding the date of the Apocalypse appears, on the face of it, fairly clear - so why would there be any reason for questioning it? Following Gentry, we can explain the conundrum as follows: the "it" in the phrase "For it was seen ... at the end of the reign of Domitian" is ambiguous as to its referent. "It" may refer to the apokalupsin (revelation) itself, so that the sentence would read, "For the revelation was seen ... at the end of the reign of Domitian."
If this is the case, then St. Irenaeus is saying that St. John saw the visions contained in the Apocalypse during the reign of Domitian, who died in AD 96. Thus, St. John would had to have written the Apocalypse no earlier than AD 96.
However, the "it" may also refer to St. John himself, in which case the sentence would read, "For he was seen not long ago, but almost in our own generation, at the end of the reign of Domitian." If this is the proper reading, then St. Irenaeus is making no statement at all as to when the Apocalypse was written - he is only referring to the fact that the last Apostle was only recently on the earth.
Which reading is the correct reading? If we take a closer look at the whole statement, the translation "for he [John] was seen not long ago" seems to make more sense. The point that St. Irenaeus is making here is that it was not necessary for the Antichrist to be revealed by name "at the present time"; and the reason he gives is that if it were necessary, then St. John himself would have "declared" the matter. He then follows this by saying, or so it seems, that St. John could have made the Antichrist known (had he wanted to) because he was still alive only decades ago, near the end of Domitian's reign.
In other words, St. Irenaeus' point is not that the Apocalypse should have made known the identification of the Antichrist, but that St. John would have made it known if he had felt it necessary. To put it another way, St. Irenaeus is emphasizing the fact that St. John lived well beyond the writing of the Apocalypse (even into the reign of Domitian), and had more than ample time to further explain the mystery of the Antichrist, had he deemed it necessary. But the fact that he did not do so is, for St. Irenaeus, proof of the fact that the revelation of the Antichrist's identity was not critical information for "the present time."
When we look at St. Irenaeus' statement in this light, not only does it make better sense of his actual words, his statement now becomes evidence for the earlier date of the Apocalypse, and not an iron-clad proof of the later date.
This reading of St. Irenaeus can be contested, of course, but this only proves the fact that his testimony is ambiguous, and could go either way.
However, when we look further into Eusebius' quoting of St. Irenaeus, we find an interesting piece of information that leans even more heavily toward the idea that St. Irenaeus supported an early date for the Apocalypse. The statement of St. Irenaeus quoted in the third book of Eusebius' Church History is quoted again in the fifth book, but this time it is prefaced with another quote from St. Irenaeus that gives us more context. In Church History 5:8:6, Eusebius quotes what he quoted in 3:18:3 - but this is what he quotes in 5:8:5:
As these things are so, and this number  is found in all the approved and ancient copies, and those who saw John face to face confirm it, and reason teaches us that the number of the name of the beast, according to the mode of calculation among the Greeks, appears in its letters ... (Eusebius, Church History, 5:8:6)
What is significant about this statement of St. Irenaeus is that he refers to the Apocalypse in terms of "ancient copies." Contrast this reference to "ancient copies" in 5:8:6 with the way he speaks in 3:18:3. In the latter passage, he is at pains to show that "it was seen" (whether referring to St. John or to the Apocalypse) very recently, "almost in our own generation." If he is indeed speaking of the Apocalypse here, as something seen "almost in our own generation", his statement about the Apocalypse existing in the form of "ancient copies" no longer makes sense. He would seem to be saying that the Apocalypse is both very ancient and very recent, which is illogical.
In fact, his reference to the "ancient copies" of the Apocalypse harmonizes quite nicely with the argument that, in the other statement, he is referring to St. John as the one who "was seen ... almost in our own generation."
At the very least, we can say that St. Irenaeus' testimony is not an open-and-shut case for the late dating of the Apocalypse. On closer inspection, he rather appears to be a useful witness for the earlier date of the Apocalypse.
St. Irenaeus' implicit witness to the early date aside, it must also be said that explicit evidence for the early date of the Apocalypse is not lacking in the ancient writings. In one of the oldest lists of the canonical books of Scripture, we read:
As to the epistles of Paul ... He wrote first of all, and at considerable length, to the Corinthians ... and then to the Galatians ... and then to the Romans ... the blessed Apostle Paul, following the rule of his predecessor John, writes to no more than seven churches by name, in this order: the first to the Corinthians, the second to the Ephesians, the third to the Philippians, the fourth to the Colossians, the fifth to the Galatians, the sixth to the Thessalonians, the seventh to the Romans. (Muratorian Canon, 3)
The key statement here is that St. Paul was "following the rule of his predecessor John" by writing to "no more than seven churches." This is a reference to the fact that, in the Apocalypse itself, St. John addresses seven letters to seven churches (cf. Rev. 2-3). What is significant here is that this text says St. Paul was taking his cue from St. John, which means that the dating of the Apocalypse roughly coincides with the epistles of St. Paul. Since we know that St. Paul was martyred in the mid 60s, during the reign of Nero, we can conclude based on this text that St. John's Apocalypse was written sometime during the reign of Nero as well - before AD 70.
Another similar statement is made by St. Clement in his Stromata:
For the teaching of our Lord at His advent, beginning with Augustus and Tiberius, was completed in the middle of the times of Tiberius. And that of the apostles, embracing the ministry of Paul, ends with Nero. (St. Clement of Alexandria, Stromata, Book VII, Cap. 27)
More plainly than St. Irenaeus or the Muratorian Canon, St. Clement says that the Apostolic teaching ended in the reign of Nero. He could not have made such a statement if St. John had written the Apocalypse after AD 70, after the reign of Nero.
Having briefly looked at the witnesses from Church History, we will conclude by reviewing the evidence from the text of the Apocalypse itself.
One of the most obvious indicators in the Apocalypse that it was written prior to the destruction of Jerusalem, and that its message is largely concerning the soon-to-be-realized judgment upon Jerusalem, is St. John's repeated use of the Greek noun tachos and the adverb tachu. No less than eight times, the Apocalypse uses these words to describe when the "coming" of Christ will take place: "The revelation of Jesus Christ, which God gave him to show to his servants what must soon take place" (1:1), "Repent then. If not, I will come to you soon and war against them with the sword of my mouth" (2:16), "I am coming soon" (3:11), "the Lord, the God of the spirits of the prophets, has sent his angel to show his servants what must soon take place" (22:6), "I am coming soon" (22:7), "I am coming soon" (22:12), "Surely I am coming soon" (22:20).
Advocates of a late date for the Apocalypse, who interpret its meaning as referring to future events, must deal with these passages. It would require quite a stretch to say that the word "soon" means "2,000 years and counting."
Another indicator in the text is found in chapter 17, where St. John speaks of the seven heads of the beast, which he says are a symbol of seven kings. The seven kings are identified as follows: "the seven heads are seven mountains on which the woman is seated; they are also seven kings, five of whom have fallen, one is, the other has not yet come, and when he comes he must remain only a little while." (Rev. 17:9-10) Note that, of the seven kings, St. John says five of them have already fallen, one of them "is" (i.e., is currently reigning), and the seventh will come later, but "he must remain only a little while."
It is no small coincidence that these seven kings align perfectly with seven specific Roman Emperors. The key piece of data here is that the seventh king "must remain only a little while." Counting from Julius as the first king, we can list the rest of the kings as Augustus (the second), Tiberius (the third), Caligula (the fourth), Claudius (the fifth), Nero (the sixth), and Galba (the short-reigning seventh king), whose reign lasted less than a year. It would be difficult to find another list of seven kings who so neatly fit the data, and thus, of significance for this study, it must be noted that it is the sixth king - or Nero - whom St. John identifies as currently reigning. This again places the date of the Apocalypse during Nero's reign, just as St. Clement and the Muratorian Canon said.
Finally, we can look at chapter 11, where St. John is given the following command: "Rise and measure the temple of God and the altar and those who worship there, but do not measure the court outside the temple; leave that out, for it is given over to the nations [Gentiles], and they will trample over the holy city for forty-two months." (Rev. 11:1-2)
The obvious significance of this passage is that it speaks of the Jerusalem temple as a currently existing structure, which was only the case up until AD 70. The statement that the outer court "is given over to the [Gentiles]", who "will trample over the holy city for forty-two months" just happens to coincide with the time of the Jewish War, when Rome (the Gentiles) ran roughshod over the temple and destroyed it. As Gentry notes, "it took almost exactly forty-two months for Rome to get into a position to destroy the Temple in the Jewish War of A.D. 67-70." (Gentry, Before Jerusalem Fell, p. 250)
If the Apocalypse is dated after AD 70, this very time-and-location specific verse must be given some other allegorical or spiritual meaning. The "temple" must be interpreted in some allegorical way, as must the "Gentiles" who will trample the outer court, and also the "forty two months". Fr. Haydock does precisely this in his commentary, stating that the "temple" is a symbol of the Church, which will be persecuted by unbelievers, but this interpretation is unsatisfying in light of what was said earlier concerning the temporal indicators ("must take place soon", "I am coming soon", etc.) that are used throughout the book.
In conclusion, we saw that advocates of the late date of the Apocalypse place a great deal of weight on the testimony of St. Irenaeus, but his testimony is inconclusive, and can even be used to support an early date for the Apocalypse. We also saw that other early Church documents explicitly testify to a date for the Apocalypse that would coincide with the reign of Nero, prior to AD 70. Finally, we looked at the internal evidence in the Apocalypse itself, and found that the temporal indicators, the enumeration of the seven kings, and the explicit reference to the Jerusalem temple, all point very strongly towards an early date for the Apocalypse, prior to the fall of Jerusalem.
The dating of the Apocalypse is critically important. If a later date is accepted, then the events predicted in the text must be aligned with some kind of futuristic interpretation, in which case the signs and symbols of the Apocalypse must be allegorized, and become open to abuse. If an early date is accepted, however, then the events of the Apocalypse point clearly to the destruction of Jerusalem, and the text can then be grounded more firmly in a literal-historical interpretation. This does not exclude an allegorical interpretation, but rather, it gives the allegorical interpretation a solid historical and verifiable grounding, which, according to the wisdom of St. Thomas Aquinas, is the sine qua non of any allegorical interpretation.