Searcher, good point.
The NAC-Peter, Jude commentary (Thomas R. Schreiner, pp.114-115) comments on 1 Peter 2:9 -
The privilege of belonging to God's people is conveyed by Peter with a number of OT allusions. Peter drew on Exod. 19:6, using the exact words found there in identifying the church as a "royal priesthood" ["kingdom of priests," NWT]. In Exodus the title applies to Israel [the whole nation, not the Aaronic or Levitical priesthood], with whom God enacts his covenant at Sinai. Israel's priesthood was such that they were to mirror to the nations the glory of Yahweh, so that all the nations would see that no god rivals the Lord (cf. also Isa. 61:6). . . Now God's kingdom of priests consists of the church of Jesus Christ. It too is to mediate God's blessings to the nations, as it proclaims the gospel. We should note the comparison and contrast here. Both Israel as a whole and the church of Jesus Christ are identified as a "royal priesthood." There is no suggestion that only a portion of Israel served as priests in Exodus 19. The difference is not the extent of the priesthood but its identity, for now the royal priesthood is the church of Jesus Christ (cf. Rev 1:6). As noted above, the priesthood here is corporate in nature, and yet this does not rule out the truth that individuals serve priestly functions. . . "Christians exercise priestly functions but always as members of a group who all exercise the same function."
[End of quote]
Compare that with 2 Cor 5:18-20. The priesthood of the Christian congregation on earth is ministering the good news to the nations. Those who serve as priests in heaven with Christ are only pictured as ministering to God's people, "the twelve tribes" of Matthew 19:28. Resurrected people become part of those "twelve tribes" by being resurrected, which, as Jesus said, makes them "children of God." (Compare Luke 20:36) Whether they remain such depends on how they act from there.
And there is the parallel with the nation of Israel under the Law. The nation had a 'priesthood' towards the Gentiles, while there was a priesthood within the nation that ministered internally.
The Christian congregation serves as a 'royal priesthood' towards the nations (compare the great crowd serving in the naos of Rev 7:15), while the 144,000 serve as priests internally during the 1000 years, when there will not be any outside nations.
The "anointing," 'preaching of the good news,' being "priests of Jehovah" and "ministers of our God" among the nations in Isaiah 61 must refer to the whole congregation, not just the 144,000, since the context of Isaiah 61 is of ministering among the nations.
The 'sealing' of the 144,000 in Revelation 7 must be something above and beyond the "promised spirit" that Christians receive as a result of their faith in Jesus. (Gal 3:14)