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There will be a great multitude of people who witness the event of the rapture, who will not believe the Satanic lies about a mass alien abduction of all the Christians, or the New Age lies about the earth ascending to a higher vibrational state, thus leaving behind all the Christians. And then after seeing the rapture, they realize the Truth of what has happened, and get right with God. When they do, they will become Saints in the Tribulation, aka Tribulation Saints. And most of them will be beheaded for their testimony.

Also others will get saved through the evangelization efforts of those who converted after the rapture, plus the efforts of the 144,000 Israeli virgins who will convert the nation of Israel, or at least a remnant of them. And then there will come the Two Witnesses who will work miracles and do the same. So yes there will be a great multitude who are saved during the Tribulation period. And all of this happens after the rapture of the Church at the end of the Church Age.

Your premise is flawed. Pre-Tribbers don't believe they are any better than anyone. We are all sinners. It's only by grace that we are saved. And it's only by grace that Yahshua(Jesus) will come again to take us home to the place He has prepared for us in heaven. We simply believe what He has promised us, His bride. If this was not the case, He would have told us so. Had He told us something different, we would believe that instead. He spoke it, we believe it. It's a matter of faith.

Many unbelievers and false Christians will witness the disappearance of all the true Christians around the world, and then they will believe. But it will be too late for the rapture of the Church for them, as the Church has already left. Then they will get saved and endure the time of the Tribulation/Daniel's 70th Week/the Time of Jacob's Trouble. And of those new believers, who will be called saints at that time in the Tribulation, who endure to then end, they shall be saved.

And of those new saints in the Tribulation, most will lose their heads, and they will go to heaven since they are saved. And in heaven, they will be given white robes and palm branches. These martyrs will come out of the Great Tribulation (the last 3.5 yrs of the Tribulation) as they are martyred and go to heaven. These will be among the wise virgins who accompany the Bridegroom (Yahshua/Jesus) and His Bride (the Church) into the Wedding Ceremony in heaven. And the foolish virgins, the false believers, will be left outside as the Bridegroom never knew them. Outside of what? Outside of the Wedding Feast in heaven during the Tribulation, and outside of Yahshua's new Millennial Kingdom on earth after His 2nd Coming with His angels and His Bride the Church.