Longtime reader St. TY asks:
Some commentators, atheists and anti-Christians, go so far as to say (mockingly) the writer of the Book was under the strong influence of some hallucinogenic substance. How should a thoughtful Christian interpret the prophesies in the Book so that it harmonises with the rest of scripture? The moon turning into blood, for example. I'm afraid we might find our "creative interpretation" of no help.
The Book of Revelation is part of a Jewish genre of literature called "apocalyptic literature", which includes the Book of Daniel, parts of Ezekiel and Zechariah, and several works which are not part of the biblical canon, such as 2 Esdras and the Book of Enoch (quoted in Jude 1:14-15). So the Book of Revelation isn't coming out of the blue. It's part of a genre which has rules and expectations (one of them being the highly symbolic visions narrated by angelic beings), which would have been understood to some extent by the original audience.
While it's obvious to anyone with an ear for literary style (excepting modern scholars) that the Letters of St. John in the New Testament were written by the same person who wrote the Gospel of St. John, it's less clear that the Apocalypse was written by the same person.
The author does identify himself as "John", and he must have been known to the original recipients. Like the Gospel, it refers to Jesus as the "Lamb of God" and the "Word of God", and has "I am" sayings, placing Revelation in a characteristically Johannine theological tone. On the other hand, the Greek style apparently is significantly different, as first pointed out by St. Dinoysius, bishop of Alexandria. I'm not a Greek expert so I can't referee this dispute, but a bunch of pro and con arguments are discussed here. Some early Christians claimed that there was a "John the presbyter" who was different from John the Apostle (the traditional author of the book).
Regardless of who wrote it, the Church decided to include it in the canon of Scriptures. So we seem to be stuck with it. In order to correctly interpret the Apocalypse of St. John, we need to bear in mind some fundamental principles:
1. The book tells us at the very beginning that it is "the revelation (unveiling) of Jesus Christ". So the main point of the book is to reveal who Jesus is (Rev 1:1). That's the real reason we know that the book is inspired, because through it, the Spirit gives us a picture of the glorified resurrected Jesus, who speaks with the same distinctive voice as the Jesus from the Gospels. "For the testimony of Jesus is the Spirit of prophecy" (Rev. 19:10).
You will notice this phrase "Lamb of God" keeps popping up. Attempts to predict the exact chronology of the end times from the symbols often get distracted from what it says about Christ and his Church, missing the point of the book entirely. It is only through Jesus that the meaning of the present and future can be seen clearly. Only the Lamb is worthy to open the Scroll which contains the meaning of history (Rev. 5). Only through his Cross can we gain the clarity to understand "what must soon take place" (1:1).
Although the imagery in Revelation is highly violent, one should notice the deep irony in the phrase "Wrath of the Lamb". Normally a lamb is something meek and inoffensive. Also notice that the sharp, double-edged sword (first mentioned in 1:16) comes out of Christ's mouth, symbolic of his words, not literal bloodshed.
2. Pretty much every single image contained in the Book of Revelation is taken from elsewhere in the Bible (often by mushing several things together). You ask how to harmonize the book with the rest of Scripture, but the book basically consists of references to the rest of Scripture! The 7 Spirits, the 4 Horsemen, the Living Creatures, the 24 divisions of Priests, the 7 Thunders, The 2 Olive Trees, the 12 Tribes, the Moon turning to Blood, the Plagues, the Beasts, the Whore of Babylon, the New Heaven and the New Earth? All blatant rip-offs from the Old Testament prophets. I'd turn these into links, but you should really figure it out for yourself. Read your Bible cover to cover, and you'll find most of these parallels on your own.
(The hardest one here is the Thunders. For this, try counting the number of times "the voice of the Lord" strikes in the lightning storm described in Psalm 29. Whatever the "voice" is saying is beyond human hearing, but that doesn't mean it doesn't have an impact on the world!)
Once you know what it meant the first time, that will give you clues about what it means in Revelation.
3. Try reading the book in light of a political satire of what was happening during the 1st century religious persecutions ("Preterism"), and also as a description of the universal experiences of the church through history ("Idealism"), as well as a prediction of what will happen during the end times ("Futurism"). These are usually presented as rival views, but I think a text like this is capable of resonating on more than one level with different events. None of these ways of reading the text should necessarily be privileged over the others, though some may work better than others for particular passages. (The 7 churches were real 1st century churches, not eras in church history, and of course the Final Judgement and the New Heaven and the New Earth are still to come.)
Ecological disasters as a result of human sin? These are, sadly, part of present day reality. You will notice these plagues are not as severe as those described in the Book of Revelation. Not yet, anyway. But beware the time of the Seventh Trumpet, when "the time has come to destroy those who destroy the earth" (Rev. 11:15).
The symbolism is fluid, many-valued. Ways of reading the book so that it is relevant only at some past time, or some future time, are sterile and implicitly contradict the idea that all Scripture is useful for growth in godliness.
The bogus crackpot methodology is "Historicism", which tries to arrange all the material into an exact chronology of historical events, identifying whatever the current author views as the greatest threat as the to-be-Antichrist. (E.g. Protestants said it was the Papacy.) Often culminating in an predicted end during (or shortly after) the writer's own lifetime, contradicting Christ's repeated injunctions not to predict the times and dates that the Father has set by his own authority. This has never worked before, and your system won't work either.
4. The numbers are symbolic, and shouldn't necessarily be taken literally.
I'd say duh, but there are too many people who, oblivious to the symbolic significance of e.g. periods of 3½ years (based on the persecution described in Daniel), try to interpret all of these as literal time periods. Maybe after substituting years for days, so they get all excited after they find two important historical events that are exactly 1260 years apart. Most people don't realize how easy it is to find coincidences like this. (It's like the birthday paradox; if you have a small number of people in a room, the expected number of pairs who have the same birthday is proportional to the square of the number of people, not the number of people. It's the same if you have a bunch of dates you want to get to line up in some pattern.)
Similarly, pretty much anyone's name can be shown to be equal to 666 if you try hard enough. (Although Nero Caeser seems like a pretty plausible guess for the original reference.) So don't do this.
5. Remember that it's a work of art as well as a prophetic text. The book is full of songs; it's about worship in the heavenly Temple which is above (which is why Tabernacle imagery keeps coming up). Christian hymns are full of references to Revelation. It's good to remember the healthy and wholesome impact which this book has had through the ages, not just the failed crackpot schemes.
There are 7 beatitudes in the Book of Revelation; the first says: "Blessed is he who reads and those who hear the words of the prophecy, and heed the things which are written in it; for the time is near." Readers often tend to assume that if we can't boil down the book to a bunch of propositions about what God is going to do, then they haven't gotten anything out of the book. But the mood, the ambiance, the tone, is just as important. God could have just given St. John a list of doctrines, but that wouldn't have been the same, would it? Notice how the book affects you as a reader, and consider that that may be the point of reading it.
Oh, and was the author on hallucinogenic mushrooms when he received this vision? That's pure speculation, as far as I'm concerned. (The claim that Patmos has hallucinogenic mushrooms / plants seems to be frequently repeated on the Internet, but after several minutes of googling I was unable to trace it back to any reliable seeming original source. Anyone know where this claim originally comes from?)
But let's suppose, for the sake of argument, that St. John had accidentally ingested a mind-altering substance. This would not necessarily prevent God from using the resulting visions to reveal a true message. God can use anything to accomplish his purposes. The brain is a chemical system, and what we do with our bodies affects our spirits.
I once knew somebody who was led towards Christianity as a result of a few hours of religious themed hallucinations after smoking cannabis. (Fact: pot can, unpredictably, cause effects akin to LSD. It can also cause permanent neurological changes. It's a much scarier drug than people make it out to be.) God may have used that, but that doesn't mean that it's a good idea to trip out on something and hope to have a religious experience. Being in an altered state of consciousness could make you more open to spiritual influences of all kinds—whether divine, or diabolical, or just weird stuff from your subconscious.
So I don't at all recommend that people take such things into their own hands. Much safer to approach God in the ways that he has appointed, through the Bible, the Church, prayers and the Sacraments. If he wants to give you a vision, he can.
Regardless of which St. John wrote the book, he was probably one of the Lord's disciples, as indicated by St. Papias. This presumably means that he knew Jesus while he was on earth. After a lifetime of suffering for Christ, and now an old man in exile, God gifted him with a vision of the same Jesus, now glorified, in order to encourage the rest of us to persist through our own suffering. The vision came because he was tripping on the Spirit. I don't think any drugs were required.