That's if I was writing a novel that tells the Revelation story. My story is based on (derived from?) Revelation but does not attempt to retell the exact same story (it's been done many times). For one thing, which interpretation of Revelation would I follow if I were to retell it? There are probably hundreds of interpretations (I own twelve of them myself, lol, from my original research). The predominant Catholic interpretation is that Revelation is a story about spiritual "warfare" and Christ's ultimate victory over evil. I do know that they generally do not believe in a real, final battle at Armageddon, which is why my final battle of book 3 will be different (Connor's forces vs Satan's forces), though still at Armageddon. Also, I was told, no "actual" war in Heaven when I once asked when in the biblical timeline it occurred. Similarly, I doubt Catholics believe in a real mark of the Beast for everyone on Earth except the martyrs. For one thing, it's simply not practical on a planet with almost 10B people. One interpretation suggested the mark was smartphones (held up to the head) and smartwatches (on the hand/wrist), as if everyone on Earth had those devices. Depending on which interpretation of Revelation you go with, things also don't happen in the order they appear in the book.
So, depending on your beliefs, things can happen out of order or don't happen at all.
Since my story is based on Revelation, I use many important elements of the Book of Revelation, but I adjust them (somewhat) to serve my plot (my interpretation, lol), without going off the deep end, except for the fact that the Antichrist ultimately ends up as the hero of the story, but that's not evident until the end of the trilogy. I end my trilogy roughly the same way as the Bible (the righteous go to Heaven, everyone else goes into the Lake of Fire, which teleports them to other worlds, from which they can never return, so they're in the lake forever). See how I did that, while still following Revelation? :-)
As for 666, since Connor overcomes his genetic enslavement to Satan halfway through book 2, there will be no opportunity for him to implement the mark of the Beast. Nor would he, since he's "Christ" not the Antichrist. So, God uses it while taking back the Vatican to mark the traitors with 666 on their foreheads. Those are the true followers of the beast, so it makes sense that at least they would be marked. You should see what God does with the corporeals (demons) at the Vatican. :-)
On a related note, I can't recall, did you read my Groundhog Day short story? It's a (revised) entry for the do-over contest about Satan's fate more or less how it will happen in the trilogy.