The Bible speaks of a "first" resurrection, implying that there will be more than one resurrection. The second resurrection will contain those who are not "the dead in Christ," but simply the dead—those millions who are not Christ's—who have not been converted, who have not heard the Gospel or understood it.
The sentence, "This is the first resurrection," refers to the resurrection to immortal life of the firstfruits of God's plan, which will occur at Christ's return, just before the 1,000-year period begins (verses 4, 6).
But notice the first sentence in verse 5: "But the rest of the dead [those who have not yet had an opportunity to understand God's truth] lived not again [would not come up in a resurrection] until the thousand years were finished."
This resurrection, which will occur after the 1,000 years, is the second resurrection. It will be a resurrection to mortal life.