The first resurrection that we read of in 1 Cor. 15 and Rev. 20, does not describe a period of time, but a class of persons having this characteristic name.
There are three divisions in verse 4 of Rev. 20.
1. “I saw thrones, and they sat upon them, and judgment was given unto them.”
2. “I saw the souls of them that were beheaded for the witness of Jesus, and for the Word of God.”
3. “And [those] which had not worshiped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands.”
The first division includes all who are taken up at the rapture and they reign with Christ. He sees not only thrones, but those who sit upon them. They share in the reign of Christ.
The second class consists of those who are slain under the fifth seal. (See Rev. 6:9-119And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held: 10And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? 11And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled. (Revelation 6:9‑11).)
The second and third classes who seem to have lost the earthly blessings of the kingdom by death, are specially named as having gained, by death and resurrection, a place in the heavenly glory with those who reign with Christ. The first of these divisions, the sitters on the thrones, have been raised or changed at the rapture. The last two are said to be in company with them, "to live and reign with Christ a thousand years." All are named "The first resurrection.” From Words of Truth