Why is Satan released after the millennium?
Satan is released after the thousand years to activate the free will of the new generations born of believers during Christ's millennial reign. He goes out to deceive the nations in the four quarters of the earth, testing their free will for a little season before the final end.
Les explains from Revelation 20 that an angel comes down with the key of the bottomless pit and a great chain, lays hold on the dragon, binds him for a thousand years, and casts him into the bottomless pit. He is shut up and sealed so he should deceive the nations no more as he had for six thousand years, until the thousand years are fulfilled. After that, he must be loosed a little season. Les says Satan is released to activate the free will of the new generations of children born of believers during the perfect thousand-year reign of Christ. At the end of the thousand years, Satan goes out to deceive the nations in the four quarters of the earth, from one end of the planet to the other.