Jesus, after he perfectly overcomes the forces of evil led by the devil, will usher in the Millennium. It will be an unprecedented time of peace and prosperity courtesy of God's Kingdom reigning on the earth. God, after this period lasting 1,000 years, will begin a time of judgment and salvation in order to finalize the fate of angels and humans.
Judgment of demons
The devil, after the Millennium, will be allowed out of his spiritual prison (Revelation 20:3). The great Deceiver will immediately set out to deceive man about God and begin sinning. Those he deceives will be gathered into a great army and try, one last time, to overcome the forces of righteousness.
God the Father's response, from heaven, will be to consume Satan's entire group of rebellious humans with fire as they prepare to attack Jerusalem (Revelation 20:7 - 9). This will mark the devil's last futile attempt to control the universe and be worshipped like a god.
What will be the final judgment and penalty given to the devil and his demons? The devil will be grabbed and tossed into the lake of fire (Revelation 20:10). This action will effectively end his spiritual existence. His fellow demons, who also sealed themselves to do evil, will also be judged worthy of death. They will also be tossed into the lake of fire (either at this time or when unrepentant humans receive the same fate). Still other spirits, perhaps those who have not sealed themselves evil, will be put into a spiritual prison (Jude 1:13).
Judgment and Salvation
God, at this point in history, remembers the countless humans who lived from Adam to Jesus' Second Coming who never fully comprehended his truth and never received his spirit. He also remembers the untold number of babies and children who either were aborted or died at a young age. His solution to offering salvation to this immense group of people is the Great White Throne Judgment.
This second resurrection, also known as the Judgment Day, is God's way of offering a FULL chance at salvation to the vast majority of man. Those brought back to life will have their minds opened to understand the Bible (Revelation 20:12). They will then have the opportunity to repent of their sins, accept Jesus as their Savior, and receive God's spirit.
The Bible hints that man, during this period of judgment, will be allowed to live a flesh-based life on earth for up to 100 years (Isaiah 65:17 - 20). Aborted babies and young children will be made alive again and allowed to grow, learn and reach their full potential. Why, however, must all those brought back to life in the future have to live a second time in the flesh?
Those resurrected must build the same kind of righteous character as all those who proceeded them. They must live a life of learning the true doctrines of the Bible and building righteous character through overcoming sin and their human nature using the Holy Spirit within them. Once God is satisfied that they have the character worthy of salvation, their names will be added to the Lamb's Book of Life and they will receive the gift of eternal life (Revelation 20:12).
The Second Death
What will God do with the relatively few humans throughout history whom, in his eyes, understood the truth but knowingly and willfully rejected it and salvation? His judgment on these people is the second death made possible by the lake of fire (Revelation 20:14 - 15). It is God's way of mercifully ending the existence of all those who committed the unpardonable sin (see Hebrews 6:4 - 6).
These judgments brought by the lake of fire are the first phase of God's plan to cleanse the universe of all sin (see 2Peter 3:7) in preparation for a perfect future!