Blessed and holy is he who has part in the first resurrection
They are blessed because they shall be priests of God and of Christ, and shall reign with Him a thousand years (Rev. Rev. 20:6+). This blessing is equivalent to the blessing for those who are called to the marriage supper of the Lamb (Rev. Rev. 19:9+) because they find entry into the kingdom of God (Luke Luke 14:15). See commentary on Revelation 19:9. Their blessing is one of seven blessings given in the book of Revelation. See commentary on Revelation 1:3.
Over such, the second death has no power
Christ promised the overcomer of the church at Smyrna, the persecuted church which experienced martyrdom, that they would not be hurt by the second death (Rev. Rev. 2:11+). They would be part of the first resurrection (at the Rapture of the Church) and the second death would have no power over them. The second death is the destiny of those who are not written in the Book of Life: Then Death and Hades were cast into the lake of fire. This is the second death. And anyone not found written in the Book of Life was cast into the lake of fire (Rev. Rev. 20:14+). But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death (Rev. Rev. 21:8+).
In the Palestinian Targum on Deuteronomy Deu. 33:6, the OT locus theologicus in rabbinic Judaism for proving the resurrection from the dead, the Targum reads: Let Reuben live in this world and not die in the second death in which death the wicked die in the word to come. In the Targum the second death means exclusion from the resurrection. Not to die the second death, then, means to rise again to eternal life.1they shall be priests of God and of Christ, and shall reign with Him a thousand years
Christ made the saints kings and priests to His God (a kingdom, priests to God , MT and NU texts) (Rev. Rev. 1:6+ cf. Rev. Rev. 5:10+). See commentary on Revelation 1:6. See Millennial Reign of the Saints.
The priestly duties of some will include service in the Temple:
I will set a sign among them; and those among them who escape I will send to the nations: to Tarshish and Pul and Lud, who draw the bow, and Tubal and Javan, to the coastlands afar off who have not heard My fame nor seen My glory. And they shall declare My glory among the Gentiles. Then they shall bring all your brethren for an offering to the LORD out of all nations, on horses and in chariots and in litters, on mules and on camels, to My holy mountain Jerusalem, says the LORD, as the children of Israel bring an offering in a clean vessel into the house of the LORD. And I will also take some of them for priests and Levites, says the LORD. (Isa. Isa. 66:19-21)Millennial Temple.