Jamieson, Fausset and Brown on 1 Peter 2:9

9. Contrast in the privileges and destinies of believers. Compare the similar contrast with the preceding context.

chosen--"elect" of God, even as Christ your Lord is.

generation--implying the unity of spiritual origin and kindred of believers as a class distinct from the world.

royal--kingly. Believers, like Christ, the antitypical Melchisedec, are at once kings and priests. Israel, in a spiritual sense, was designed to be the same among the nations of the earth. The full realization on earth of this, both to the literal and the spiritual Israel, is as yet future.

holy nation--antitypical to Israel.

peculiar people--literally, "a people for an acquisition," that is, whom God chose to be peculiarly His: Ac 20:28, "purchased," literally, "acquired." God's "peculiar treasure" above others.

show forth--publish abroad. Not their own praises but His. They have no reason to magnify themselves above others for once they had been in the same darkness, and only through God's grace had been brought to the light which they must henceforth show forth to others.

praises--Greek, "virtues," "excellencies": His glory, mercy (1Pe 2:10), goodness (Greek, 1Pe 2:3; Nu 14:17, 18; Is 63:7). The same term is applied to believers, 2Pe 1:5.

of him who hath called you--(2Pe 1:3).

out of darkness--of heathen and even Jewish ignorance, sin, and misery, and so out of the dominion of the prince of darkness.

marvellous--Peter still has in mind Psa 118:23.

light--It is called "His," that is, God's. Only the (spiritual) light is created by God, not darkness. In Is 45:7, it is physical darkness and evil, not moral, that God is said to create, the punishment of sin, not sin itself. Peter, with characteristic boldness, brands as darkness what all the world calls light; reason, without the Holy Spirit, in spite of its vaunted power, is spiritual darkness. "It cannot apprehend what faith is: there it is stark blind; it gropes as one that is without eyesight, stumbling from one thing to another, and knows not what it does" [Luther].