In the Old Testament tabernacle (and temple) no one was permitted into the proximity of God (who dwelt in the Most Holy Place) except priests. Their specific function was to mediate between the people and God through officiating at the sacrifices (this office, of course, required their undivided devotion to God). We might say that they had God’s ear and could speak to God on behalf of the people. In the present passage, martyrs were given thrones so as to reign with Christ. That they are now called “priests of God” indicates that Christ’s coregents have the ear of God as they speak to him about developments and needs in his kingdom, including how fellow believers remain objects of the beasts’ attacks.
6 Blessed and holy is the one who shares in the first resurrection! Over such the second death has no power, but they will be priests of God and of Christ, and they will reign with him for a thousand years.