After Jesus returns to earth and establishes his kingdom, there will be two groups of people on earth — those who have been made immortal and those who are still mortal.
The immortal saints are those who have been faithful believers and are welcomed by Jesus into his kingdom (Matthew 25:34-40,46). They will assist Jesus in the administration of the kingdom:
But the saints of the Most High shall receive the kingdom and possess the kingdom forever, forever and ever.
And the kingdom and the dominion and the greatness of the kingdoms under the whole heaven shall be given to the people of the saints of the Most High; their kingdom shall be an everlasting kingdom, and all dominions shall serve and obey them.’ (Daniel 7:18,27)
See also 2 Timothy 2:12; Revelation 5:10; 20:6.
The mortal population are those who survive the events surrounding the return of Christ, but are not judged and so are not granted either eternal life or eternal death. These are those who are mentioned in the following verses, for example.
No more shall there be in it an infant who lives but a few days, or an old man who does not fill out his days, for the young man shall die a hundred years old, and the sinner a hundred years old shall be accursed. (Isaiah 65:20)
Then everyone who survives of all the nations that have come against Jerusalem shall go up year after year to worship the King, the LORD of hosts, and to keep the Feast of Booths. And if any of the families of the earth do not go up to Jerusalem to worship the King, the LORD of hosts, there will be no rain on them. (Zechariah 14:16-17)
And when the thousand years are ended, Satan will be released from his prison and will come out to deceive the nations that are at the four corners of the earth, Gog and Magog, to gather them for battle; their number is like the sand of the sea. (Revelation 20:7-8)