After the judgments that follow the Second Coming, believing survivors from the Great Tribulation will go into the Millennial Kingdom in their natural state and have children. Of those millions of children, some will believe and be saved; others will not. During the thousand years, many believers and nonbelievers will die. What will happen to their souls after death? They can’t go to the New Jerusalem. So, where will they be until the Great White Throne resurrection and judgment?
The Bible says that at the end of the Millennium the unsaved dead from all ages will be raised up for judgment I believe this will include the unsaved dead from the Millennium. The part of Hades that houses the unsaved dead will remain in use until the Great White Throne judgment when Hades will give up the dead in it (Rev. 20:13).
But nothing is specifically said about what happens to believers who die during the Kingdom Age. Of all of mankind, the Millennial Saints, as they’re sometimes called, is the group we know the very least about. The Bible does not mention where they will go when they die.
There is one hint that believing Millennial saints who have died may be present at the Great White throne judgment. Rev. 20:15 says, “If anyone’s name was not found written in the book of life, he was thrown into the lake of fire.” I don’t believe John would have said that if there will not be any believers at that judgment, because it’s obvious that no unbelievers’ names will be found in the book of life.