Why is Satan released at the end of the millennium?
Satan is released at the end of the millennium so that the new generation born under Christ's benevolent rule, who have never had to choose for or against God, can exercise their free will. Giving a genuine choice requires more than one option, so Satan returns to present his temptation, just as he did to Eve.
Les explains that during the millennium, children born to believers will have no opportunity for out-and-out rebellion because no satanic force is operating and Christ rules as King with absolute benevolent power. Yet these children are still born with the same old Adamic nature. So at the end, Revelation says Satan must be released for a little season. Why? This new generation has never had to make a choice for or against God, and they must be brought to a place of choosing. They have to exercise their free will, just as everyone else did. Les notes that giving a choice requires more than one option. Therefore Satan is brought back for the sole purpose of giving these new generations the opportunity to choose. He is given full opportunity to present his options, just as he did to Eve in the garden. It will be the same type of temptation, because the Kingdom is so glorious—who could want anything different or better? Eve had it that good, yet Satan's ploy was to tempt her with 'You can be like God.' Les thinks the same thing will happen: Satan will tempt the new generations with the concept that they have it perfect but are still not God.