Why is there no sin during the Millennium?
During the Millennium, Satan is locked up and removed from the scene. Though children are still born with the Adamic nature, there is no tempter present to activate it, so there is no real rebellion and no sin. The Adamic nature remains inoperative for the thousand years.
Les explains that the Kingdom Age will have flesh-and-blood people who marry and have children for a thousand years. There is no death to speak of, no sickness or disease, and conditions will be much like the Garden of Eden, producing a tremendous population explosion. The believers who enter are safe for eternity, but their children are still born with Adamic natures. Then, as Revelation 20 shows, the first thing God does is lock up Satan. Without Satan on the scene to test those Adamic natures, there is no real rebellion and no sin; the Adamic nature just lies inoperative because nobody activates it. For a thousand years it will be glorious and utopian, yet it is also a dispensational period. Les contrasts this with Adam and Eve leaving the Garden, when circumstances changed and the curse brought sweat, weeds, thistles, sickness, disease, war, and famine.