Why will Satan test the new generations after the millennium?
Satan will be loosed after the thousand years to test the new generations that come on the scene in that kingdom economy. This final testing is necessary because the millennium is the seventh period of testing in human history, and God always does things in seven, so there must be a seventh testing of the human race.
Les explains that Satan cannot be sent to the Lake of Fire yet because that is a place of eternity, and Satan is not ready for eternity. He has one more opportunity at the end of the thousand years. God confines Satan in the bottomless pit, shut up and sealed, so he should deceive the nations no more until the thousand years are fulfilled. After that, he must be loosed for a little season. Then he will have full freedom to deceive the nations that will be back on the scene once again and to test the new generations that will come on the scene in that kingdom economy. The millennium thus becomes another period of testing. Human history has already had six periods of testing, and this one must be the seventh because God always does things in seven, so there has to be a seventh period of testing the human race.