But the rest of the dead lived not again until the thousand years were finished; should be finished. Omit "but;" omit "again." It is important to notice the omission of "again;" the rest of the dead lived not until, etc.
The best explanation of these words seems to be that the "rest of the dead" refers to those Old Testament saints and others (such as godly heathens) who were in the world before Christ's act of atonement—"the thousand years" (see on Revelation 20:2, above)—had been accomplished.
They could not be said to have lived, in the high sense in which St. John uses the word, not having known Christ; for "in him was life" (John 1:4; John 5:40, etc.). But by Christ's redeeming work, these were placed on a level with Christians (cf.
Luke 7:28, "John the Baptist: but he that is least in the kingdom of God is greater than he;" also Hebrews 11:39, Hebrews 11:40, "And these all, having obtained a good report through faith, received not the promise: God having provided some better thing for us, that they without us should not be made perfect").
This is the first resurrection. These words refer both to the reigning of those mentioned in verse 4, and to the living of those in verse 5 (vide supra). This "first resurrection" is the spiritual rising with Christ, which is a consequence of his redeeming work.
It is to be noticed that St. John nowhere makes use of the phrase, "second resurrection," though he does use the words, "second death." Both the "first resurrection" and the "second death" are spiritual operations.