Well, this is my private interpretation. The Fathers were not stumbling all over themselves to interpret Revelations, and I pretty much gave up studying the book after converting to Orthodoxy. So, there really is no Patristic backing for what I have to say.
I believe that the key to this verse is in the passages before it. I believe that this passage refers to the Antichrist, and that he will be given power to wage war against the Saints, and will overcome them (but not prevail against them). While I believe that Salvation is a process, I also believe that God, who knows everything, also knows who will be saved. And it is those names that are written in the "Book of Life". What this passage means is that the Antichrist will be universally worshiped, and this includes the Orthodox, who will apostatize. He will subdue the Church, just as the true Jewish Church was subdued during the time of the Great Prophet. However, like that time, there will be a remnant of people who will not bow their knee to the Antichrist, just like there was a large number (7000) in Israel that did not bow to Baal. These are those who's names are written in the book of life. While the Church will be overcome, and the visible Church will cease to exist, the Devil and the Antichrist will not prevail since the remnant will remain until Christ comes again. The remnant will be small since Jesus says "When I return, will I find Faith?"
This is just my opinion, for what it is worth.
"The Orthodox, who will apostatize", or "the Orthodox who will apostatize"? The comma makes a big difference, so I'm trying to figure out what exactly you mean.
You are correct, and I wrote it as I intended. Although, I can see by your question what you are really asking. When I used the term "Orthodox" in this context, I meant the visible Church. To apostatize, the people would first have to be true believers, part of the Body of Christ. What will be left will be individuals, just like during the great Apostasy in Israel. Granted, if these individuals are written in the Book of Life, they will be Orthodox, even if they do not form a visible and coherent congregation. However, they will not cease to be part of the Body of Christ. My understanding, and that of my Priest, is that the Orthodox Church that we know today, as an organization, will cease to exist as a Christian entity, but will serve the Antichrist. I know this is controversial, and that is why I offer it as a private opinion, although this opinion has been shared by several Priests and Monks that I have known in both the ROCOR and the Serbian Churches.
There may also be Christians that are not "Orthodox" in this group, since Christ Himself says "and many sheep have I also that are not of this fold". I have always understood the "fold" to be the visible Orthodox Church here on Earth, and that Christ had sheep (who's names would be in the Book of Life) outside of the Church. We do not know who they are because there is no Salvation guaranteed outside the Church, and they do not exist as an organization such as the Orthodox Church. It is possible that the remnants of the overcome Orthodox Church, mixed with the "sheep not of this fold" will comprise the few that will have Faith at the Second Coming of our Lord. It will be a time such as that of the great persecutions where pagans such as Roman soldiers were Baptized into the Church by their own blood as the repented and converted after watching the death of the Christians. These will not have had time to enter into the Church by the means currently in place, just as they did not in those days.
Again, I offer this as an opinion and not a teaching, and I stand prepared to recant these opinions if they are found heretical by the Church.