Which specific believers will enter the millennial kingdom?
Only believers enter the millennial kingdom. Specifically, Gentiles who believed during the Tribulation through the preaching of the 144,000, along with the remnant of Israel who are saved, enter as flesh-and-blood people of the Kingdom Age. They will have children throughout the thousand years, though those children are born with Adamic natures.
Les explains that when believers come into the Kingdom, it is only believers. The Gentiles who believed during the Tribulation under the 144,000's preaching, and the remnant of Israel who are saved, come up and become the flesh-and-blood people of the Kingdom Age. They will have children for a thousand years. Because there is no death to speak of, no sickness or disease, conditions will be much like the Garden of Eden, producing a tremendous population explosion. Those who entered as believers are safe for eternity, but their children will still be born with the Adamic nature. Les notes that God locks up Satan, so without Satan to test those Adamic natures, there is no real rebellion or sin and the Adamic nature lies inoperative. Thus the thousand years will be glorious, a utopia, yet also a dispensational period.