1)
Will there be sin and death in the Millennium?
Unfortunately, yes (Isaiah 65:20-25). Although every one entering the Millennium is loyal to Christ, by the end of it, some of their rebellious offspring declare their loyalty to Satan and follow him in a futile attempt to overthrow His followers and Jerusalem (Rev.19: 15; 20:7-9).
2) During the 1,000-year reign of Christ, will there be marriage and reproduction?
Yes. Millions who are converted to Christ and survive the Tribulation will enter the Millennium in their human bodies (Rev. 7:9-17). Given the idyllic conditions and long life, they will repopulate the earth (Is. 65:20-25; Zech. 8:5).
3) If few are saved, are many of the people entering the Eternal Kingdom children born during the Millennium?
It is speculated that there will be multiplied millions of babies born during the Millennium, the vast majority of whom will put their faith and trust in Christ, the reigning King. They will enter the eternal state as part of the remnant of Israel.
4) How will it differ from the present earth and the Eternal Kingdom?
Presently, Satan is the "god of this world" and exerts enormous influence on human beings to live sinful lives of rebellion against God.
During the Millennium, Christ rules the Earth from Jerusalem. Sin, rebellion, and death will be the exception.
During the eternal state, sin and Satan are gone forever. Christ will rule the perfected saints in the perfect place, the New Jerusalem (Rev. 21:22-27).
