Why is the thousand-year reign of Christ a time of testing?
The thousand-year reign is a time of testing because generations born during it still carry the Adamic nature but have no need of salvation under the King. At the end, Satan is loosed to deceive them, forcing a choice and proving humanity's makeup is incorrigible.
Les explains that although the thousand years of Christ's reign are glorious and there has been no sin, it is still a time of testing. The new generations born during that period still have the Adamic nature, yet they have had no need for salvation because they are under the rule of the King. As the end approaches, they must make a choice. The whole purpose of the thousand years is to prove once again that the human makeup is incorrigible. Just as Eve was tempted, these people in a perfect environment will be tempted when Satan is loosed in Revelation 20:8 to deceive the nations in the four quarters of the earth, gather them to battle, and then fire comes down from God and devours them.
And when the thousand years are expired, Satan shall be loosed out of his prison,