What the Reign of the Saints with Christ for a Thousand Years Is, and How It Differs from the Eternal Kingdom
“The rest of them,” he says, “did not live.” For now is the hour when the dead shall hear the voice of the Son of God, and they that hear shall live; and the rest of them shall not live. The words added, “until the thousand years are finished,” mean that they did not live in the time in which they ought to have lived by passing from death to life. And therefore, when the day of the bodily resurrection arrives, they shall come out of their graves, not to life, but to judgment, namely, to damnation, which is called the second death.
For whosoever has not lived until the thousand years be finished, i.e., during this whole time in which the first resurrection is going on—whosoever has not heard the voice of the Son of God, and passed from death to life—that man shall certainly in the second resurrection, the resurrection of the flesh, pass with his flesh into the second death. For he goes to say “This is the first resurrection. Blessed and holy is he that has part in the first resurrection,” or who experiences it.
Now he experiences it who not only revives from the death of sin, but continues in this renewed life. “In these the second death has no power.” Therefore it has power in the rest, of whom he said above, “The rest of them did not live until the thousand years were finished;” for in this whole intervening time called a thousand years, however lustily they lived in the body, they were not quickened to life out of that death in which their wickedness held them, so that by this revived life they should become partakers of the first resurrection, and so the second death should have no power over them.
Source: City of God (New Advent)