@Duran
The first three chapters of Revelation are about the Church. After chapter 3, the Church (all born again believers since Pentecost) are not seen again for the rest of the book (Tribulation) until Christ returns to defeat the Anti-Christ and accept the throne (now whole-heartedly) offered to him by his brothers - the Jews. So, all of Revelation after chapter 3 refers to various people separate from Church-Age believers.
The first time he came was to be a curse, in our place. So, the people cursed him by murdering him. "Cursed is every man who hangeth upon a tree". " He who knew no sin became sin for us, so that we might become the righteousness of God". His loss was our incredible gain. Christians trade places positionally with Jesus. This is a very favored position, in exchange for nothing more than trusting Jesus.
So, like you asked: If co-called Christians are gone during such time, why does it call for endurance and faith for them during that time? And how does the beast/8th king wage war and conquer them, if they are not here?
Just because Christians are evacuated out during the GT doesn't mean that God has stopped being in the business of saving people - far from it.
1. People who become believers after the Rapture are commonly referred to as Tribulation Saints. There is some debate as to whether or not these believers make up part of the "Bride of Christ" or not. But, it is clear that many believers will emerge on earth at this time but will likely be killed for their faith in Jesus. They are not forgotten since they became family members too. All that is left of "The Church" on earth at this time is the empty husk of Christianity, The Laodicean Church. The real Church will be gone, in heaven having the biggest party you can imagine. But, Jesus will still save individual believers who put faith in him...as he always has. People were always saved by faith regardless of the time period they lived in.
2. Then, there are Jews who put faith in Christ after Jesus raises up the real 144K male Jewish virgin evangelists and also sends the "two witnesses" to Jerusalem (Likely Moses and Elijah). If you'll recall, they were previously raptured to Heaven, in the case of Elijah, rather dramatically in a chariot of fire. Remember: "All Israel will be saved" ... that is, ALL those that are still alive at his return. A remnant will be saved.
3. Then, there is perhaps 1/4 of the general population that are still alive on earth after all the wars of the Anti-Christ. Jesus makes it clear that if they did not go along with the murderous plan of the A/C to murder all the Jews, then as King he welcomes them into his Millenium Reign, which will be global, but will still have many nations under his jurisdiction -
- And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me. Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world. - Jesus
As you can see, this commonly accepted endtimes scenario is vastly different from the WT .God is in the business of saving people all they way to the Millinial Reign and during it as well. People on earth during the Thousand Year Reign will still have their sin natures to deal with. Christians on the other hand are totally glorified in their soul, body and spirit after Glorification. Right now, a Christian only receives a new spirit at his moment of Salvation - a "down payment" or, "ernest" money as the King James puts it. Christians still deal with the sin nature in their bodies and in their soul (minds). But at the Rapture, Christians will have the sin nature permanently removed forever, receiving a purified soul and a glorified body (like the body Jesus received after his resurrection) .... thank God!