At the end of the tribulation, there will be a judgment of Israel and a judgment of the Gentile nations. The judgment of Israel is described in Ezekiel 20:34-38, which states:
Ezekiel 20:34-38 And I will bring you out from the people, and will gather you out of the countries wherein ye are scattered, with a mighty hand and with a stretched out arm, and with fury poured out. And I will bring you into the wilderness of the people, and there will I plead with you face to face. Like as I pleaded with your fathers in the wilderness of the land of Egypt, so will I plead with you, saith the Lord GOD. And I will cause you to pass under the rod, and I will bring you into the bond of the covenant: And I will purge out from among you the rebels, and them that transgress against me: I will bring them forth out of the country where they sojourn, and they shall not enter into the land of Israel: and ye shall know that I am the LORD
Notice that the Lord will plead with them face to face (v.35-36). This is a description of a judgment where they are physically present before God. The time is indicated as being when God gathers them from all the countries where they are scattered (v.34). This is clearly a judgment of Israel after the tribulation. Those who are rebels will be purged (v.38); that is, sent into judgment. Those that are right with God will be brought "into the bond of the covenant" (v.37). As taught in Romans 11:26-27, the covenant is that God will take away their sins. These Jews will enter into the land (compare v.38) and will clearly take their place in the kingdom during the thousand year reign of Christ.
The judgment of the Gentiles at the end of the tribulation is described in Matthew 25:31-46. The righteous Gentiles will receive this call: "Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world" (v.34). They enter into the kingdom age alongside the righteous Jews. This explains the kingdom of Daniel 7:14 that is made up of "all people, nations, and languages" and the "many nations" that come to Jerusalem as seen in Micah 4:2.
The righteous who go from the tribulation into the kingdom age will still have human bodies. Evidently, they will have children and multiply the population of the earth (see Jeremiah 30:19-22; Ezekiel 47:22). Although those brought into the kingdom are sealed by the covenant and their sins are forgiven, their children will be born with a sin nature since they are still physical descendents of Adam. This explains a number of things about the kingdom age. It will need the strict enforcement of the law of the kingdom as seen in Matthew 5-7. Even with this strict enforcement, nations will at times rebel and will need to be brought into line by the withholding of rain (Zechariah 14:16-19). Christ as king will rule with a rod of iron (Psalm 2:7-9)--an unnecessary act if all willingly submit. And, finally, at the end of the kingdom age the devil will be able to muster an immense army of rebels against the Lord (Revelation 20:7-9).