The Great White Throne Judgment is an event that follows the Millennial Kingdom and involves the once and for all final judgment of all the lost and unsaved who have been sent to Hell and await their final punishment. This won't be a question of who will be there or where they will go, as that will already have been decided by then. But rather this will be a determination of the degree and severity of the punishment each will receive with some receiving greater punishment, and others less. But all who stand at this judgment will be punished regardless. It will also signal the final purging of all evil, wickedness, sin and disobedience from God's kingdom, leaving only the saved, redeemed, obedient servants of God, along with the Holy Angels, to fill His kingdom. The actual judgment itself will be overseen by Christ Himself. After each lost soul is judged, they will then be cast into the Lake of Fire where they will begin serving out their punishment.
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The Judgment Seat of Christ
Daniel 7:9-10