The rest of the dead did not come to life UNTIL the THOUSAND YEARS were completed.
This verse shows the contrast to "they came to life" in the previous verse. The "rest of the dead" are the ones who now are receiving their wages (Romans 6:15-23). Specifically, they took the mark of the beast and committed murder (Exodus 20:2-13-17). John shows us that these people will remain dead, probably both physically and spiritually until the 1000 years are COMPLETED.
This is the FIRST RESURRECTION.
This one phrase has caused more controversy among scholars than any other in the Revelation. Translators point out that the verb "is" may more properly be translated "completes". In any case, FIRST RESURRECTION implies a sequence. Even though it is not specifically stated, the SECOND RESURRECTION, implied, is generally taken to be the one in verse 12.
Now, back to the FIRST RESURRECTION. We pointed out in the previous verse, that "they", the ones of the FIRST RESURRECTION, occupy the thrones. We probably will never know who "they" are, but we have a couple of hints from scripture who "they", the joint heirs, might be.
As 1 Corinthians 15:20-28 points out, this is a sequenced RESURRECTION. Christ started the process when He conquered death and RESURRECTED from the tomb/death (Luke 24:1-12). Then Matthew 27:51-52-56 records a curious (to us) RESURRECTION of saints in Jerusalem. This RESURRECTION may also have occurred throughout the whole area, but only Jerusalem is noted where this occurred. There is no record of these bodies going back into their tombs/graves. The "many bodies" could be interpreted to be all "joint heirs" from Adam and Eve until Calvary. Next the "dead in Christ shall rise" as 1 Thessalonians 4:16-17 teaches.
In the middle of the tribulation, the two witnesses are RESURRECTED (Revelation 11:11). And from the previous verse in this chapter of Revelation, the tribulation saints complete the FIRST RESURRECTION. The important point to understand is that the "rest of the dead", those who are spiritually dead (Ephesians 2:1-4-6-10) will not be around during Christ's millennial reign. Active evil now is dormant.