The term could refer to those who have physically died over the centuries of the world’s existence. In that case we are left with the question of what happens with the billions who are alive when God sets up his throne for judgment on the Last Day. The term could also be descriptive of all mankind (the living and the dead) since we are all dead as a result of our sin (Genesis 2:17; Romans 6:23; Ephesians 2:1; 1 Timothy 5:6) and hence all rightly awaiting God’s judgment. The latter understanding of the term “the dead” fits well in this passage.
12 And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done.