Ok, I realize this is somewhat confusing in scripture, but if you look at Daniel 7, when Jesus returns to reign He destroys the dominions or nations, but some of these remain for a season. Here is the passage:
Then I kept looking because of the sound of the boastful words which the horn was speaking; I kept looking until the beast was slain, and its body was destroyed and given to the burning fire. As for the rest of the beasts, their dominion was taken away, but an extension of life was granted to them for an appointed period of time. Daniel 7:11-12
Obviously, these are not eternal beings, but human beings. And Jesus when He Judges The Nations in Matt 25 is doing this after He returns. When He returns all the saved of this age are with Him, resurrection and Rapture of the saved is over. These nations are therefore being judged as scripture tells based solely on their works or as it states here based on how they treated His brethren. We His brethren are standing here with Jesus. So there are three groups there. In Joseph Goods book on the kingdom from a Messianic Jewish perspective, he says that the Jews believed that there were three peoples to be judged at the Lords return. Or in the case of non Messianic Jews at His Coming.
My understanding of this would be — The totally righteous, that would be us who are in Christ, the totally wicked, the goats could represent these, and those who are given until Yom Kippur the day of atonement to repent or be cast out would be the sheep nations. Remember not all sheep are saved, Jesus leaves the ninety and nine to go after one lost sheep. Remember these sheep nations have not yet died nor have they been Raptured, so they are not redeemed. The eternal kingdom was prepared for them to inhabit in the flesh, they are there because they have done righteous deeds to us who were here during Tribulation, and now they must accept Jesus as their savior by Yom Kippur. Even though the kingdom is eternal, and they are judged righteous for their deeds, they cannot earn the kingdom for eternity based on their own righteous deeds. Their is nothing mentioned in the Matt 25 passage that indicates that they even know Jesus. If they were saved would they not know when they had given Jesus food, drink, or clothing by doing it to the least of these “their” brethren? But Jesus didn’t say that these who they helped were “their” brethren, but His brethren. These fit with Daniels dominions who are given life for a season. Beyond that I cannot speculate or know Gods mind, because He has not revealed any more in scripture. But you may also consider the nations who remain in Zachariah 14. Neither do these seem to be saved nations, but instead nations who are ruled by Christ. Remember He rules with a rod of iron, and we the saved sons of God rule with Him for 1000 years. He rules over nations (peoples) that are unsaved, or who are in the process of being saved.
I hope that this helps, because there is much confusion on this topic.