Are you saying you think that they will believe in Him when He returns? If so, that is completely false. It will be too late for anyone to repent at that point. He will be taking vengeance on all unbelievers at that time (2 Thess 1:7-10) regardless of whether they are Jew or Gentile.
It is not about repentance at that point. It is about God changing hearts and minds. God can redeem whom God desires. God literally redeemed the whole of Adam's flesh and blood. The vast majority rejected that redemption. If you allow God to accept that rejection without prejudice, then why the Atonement and the Lamb's book of life?
Your logic comes from an empty Lamb's book of life, and at the end some names will be written down. Even at the GWT names will still be there for all those who spent time in sheol. You may not agree on second chances. But all excuses are removed at the GWT. The vast majority will still be removed at that point.
So many alive today will be in the Lamb's book of life. They are of Israel. They are not the church. They have been redeemed since before creation. They have yet to have the right point of view in their heart and minds.
No one can determine another individuals final state. Only God knows that. Why do some immediately jump to the judgment that God cannot forgive sin based on a person's rejection of that forgiveness? God was the full Atonement knowing full well many would reject that Atonement.
I have even heard in the past that the more people who became saved would make the Cross less of a burden, but is that really the case? All in Adam were equally sinners, so the Atonement was full on the Cross, even though some would accept that Atonement. The Cross was completed before creation, not after many accepted and were made whole. The Atonement still covered even those who were made whole the same as those who were not made whole. So even after the Second Coming God can change hearts and minds as He already made that possible despite each individuals choice.
John Calvin allegedly figured this out, but his followers went out of their way in the opposite direction claiming those covered by the Atonement were already fixed, and no one could ever choose period. God did all the choices. Your point about the total opposite is not true either. You think Matthew 25 and the sheep and goats is based on human choice. It is not. Both the sheep and goats were clueless and never chose one way or the other. This is the moment where all of Israel alive at that point realize who their Messiah is, and sadly many will still be singled out as goats by God's choice. Atonement is not by works, yet Jesus claims works is the metric mentioned. All of them had been redeemed on the Cross. Yet the goats still were awarded eternal damnation because their works could never save them to begin with. You can judge those sheep all you want, but to no avail. God chose them, and changed their hearts and minds. Literally, the only ones who ever existed, who did not choose as Calvinist would point out. Do you think some sheep at that point would dare stand up and ask to be a goat? They just were saved, why would they suddenly loose their mind?
Sure it is easier to just generalize this is just all the saved and lost. But then you base righteousness on works. Also tell me why all the redeemed are dead standing at the GWT. The Atonement proclaimed all in the Lamb's book of life redeemed before creation. Nothing removed any one causing them to be dead at the GWT instead of glorified in Paradise. One cannot generalize the GWT. It is for the dead only. The same for Matthew 25. The sheep and goats are for Israel explicitly. Israel was explicitly called out of the other nations and God's choice. Starting with Abraham and Isaac. Then specifically with Jacob. The church meets the Lord in the air. The church is not brought before the throne set up in Jerusalem.