Why does Satan have to be released after the thousand years?
Satan is released after the thousand years to prove the innate rebelliousness of the Adamic nature. The children born during the millennium to believers who entered at its beginning never had to make a choice, so Satan is released to test those new generations, and they will follow him.
Les explains that after Christ has accomplished all things—pouring out wrath, returning, setting up His glorious kingdom, bringing in the resurrected of all ages to rule and reign, and ruling over a new generation repopulating the planet—there is a thousand years of perfect rule with no poverty, war, depressions, or anything amiss. Yet to prove the innate rebelliousness of the Adamic nature, the children born to believers who came in at the front end, who never had to make a choice, must be tested. That is why Satan has to be released. Satan comes for that specific purpose: to try and test these new generations who have never before made a choice, and again they will follow Satan.