Why will the one confined during the thousand years be released after those thousand years?

Les explains that the one confined for the thousand years must be released afterward in order to test the final population of that thousand-year time. He ties this to the end of the thousand-year period, when that final population needs testing. The following verse describes thrones, rule given to those seated, and the souls of those beheaded for the witness of Jesus and for the word of God, who had not worshipped the beast or his image and had not received his mark; they lived and reigned with Christ a thousand years.