above all mountains, and all nations will be going to it, and God will teach us of his ways, out of Zion will go forth the Law. Since it says all nations, this will include also nations that became extinct before any of their people even heard of God, much less been taught. So it will be the time after all people will be resurrected. The same prophecy was revealed also to Micah, see Micah 4:1-3, so it is an important prophecy, as it was revealed to two prophets.
Isaiah also reveals in 26:9-10 that when God's judgment dawns on the earth, the world's inhabitants will learn justice. But the wicked person will not learn justice, he will act perversely. So we can see that when we will be resurrected, the people will learn justice but the wicked will refuse to learn justice. This will be a time of judgment, as we see also in Is. 2:4 and Micah 4:3.
Zech. 14:16-19 prophesies that people of various nations will be coming to Jerusalem for the feast of Tabernacles to worship God, and if some families refuse to come, they will be punished by drought or plague, and this will include the Egyptians. So clearly worshipers who used to be Egyptians will no longer be associated with Egypt.
In Mt. 25:31-46, Jesus is telling us that when he will come in his glory, and people will be divided in 3 groups, the brethren of Jesus, the sheep, who will be helping the brethren, and the goats, who will receive the eternal punishment. The brethren of Jesus are clearly the people who will die saved. The sheep in that story will be the people who will be saved after they are resurrected, and the goats are the people who will never be saved. So this will be the time of judgment, after we are resurrected, this will be clearly happening during the millennium, as the millennium will be the time when this new Law will be in effect, when the feast of Tabernacles will be mandatory for everyone, not like now, when it is not mandatory for anyone, or the time of the Law of Moses, when going to Jerusalem for this feast was mandatory just for Israelite males, not for women and not for non-Israelites. During that time God's temple will be rebuilt in Jerusalem, on Mt. Zion, and we will have sacrifices, as we can see in Ezekiel 43-48. But the sacrifice laws will not be identical to those in the Law of Moses. So this will be during the millennium, since following the millennium there will be no temple in the city, see Rev. 21:22.
There are already hundreds of Christian denominations that believe that the unsaved will have the opportunity to become saved after they are resurrected. Perhaps the first one in modern times who restored this biblical truth was the preacher Charles Taze Russell, who preached in the second half of the 19th century, until he died in 1916, he founded the International Bible Students Association. After he died, there were divisions in the association, they got split up into different denominations, the largest group was later renamed the Jehovah's Witnesses and later they became apostate, but they still teach similar doctrines about the resurrection and second chance for salvation after the resurrection. But in some doctrines they went far from what Russell was teaching. Christian denominations that teach basically the same as what Russell was teaching include for example the Dawn Bible Students Association, the Laymen's Home Missionary Movement, and a few smaller groups.
A somewhat similar teaching about conversions after resurrection was taught by Herbert W. Armstrong, who founded the now former Worldwide Church of God, but he placed the conversions at a time after the millennium, and I believe he was in error in this. After his death the Worldwide Church of God started changing many doctrines, became basically an average evangelical denomination, but some people stayed faithful to Armstrong's doctrines, though they might disagree on details, so some dozens of denominations basically faithful to Armstrong were founded, the largest is possibly the Philadelphia Church of God, which is named for the faithful church of Philadelphia in Asia Minor to whom Jesus dictated his letter for John to send it to Philadelphia, in Rev. 3. There are also some denominations which have made larger modifications to Armstrong's doctrines but still have similar doctrine about salvation after resurrection.
There are also a few denominations that teach a somewhat different doctrine, they teach that the dead are not totally dead, their souls are alive and they can be taught the gospel, they include for example the New Apostolic Church, and some offshoots from that church, and of course most famously this is taught by the Church of Jesus Christ of Latter Day Saints and some of their offshoots, but they went into apostasy, so I would not recommend those Mormon churches. There are also some Evangelical pastors and theologians that teach a similar doctrine, but have not founded denominations, but might belong to larger denominations, in which their views are a minority, as most Evangelicals still unfortunately do not believe in salvation after death. Anyway, since the Bible teaches clearly that the dead are totally dead until the resurrection, I believe that such a view of conversions after death and before resurrection is in error.
Now you ask why then bother? Well, people who are not saved, even if they are mostly good people, they do not have God, if they pray to God, then he will not listen to their prayers, so their prayers are not answered, for example they can pray for healing but no supernatural healing will come to them. So it is beneficial to get to know God and become saved, then God loves you and can answer your prayers, even if the answer is not quite what you requested. So sometimes there is supernatural healing, but sometimes the answer is no healing for the disease, you just have to endure, finally you will die, and then you will be resurrected healed, when Jesus comes back.