Albert Barnes Commentary 1 Peter 2:10

"who in time past were no people, but now are the people of God: who had not obtained mercy, but now have obtained mercy." — 1 Peter 2:10 (ASV)

Which in time past were not a people. That is, who formerly were not regarded as the people of God. There is an allusion here to the passage in Hosea 2:23, And I will have mercy upon her that had not obtained mercy; and I will say to them which were not my people, Thou art my people; and they shall say, Thou art my God. It is, however, a mere allusion, similar to how one might use another person's language to express their own ideas, without meaning to imply that both passages refer to the same subject.

In Hosea, the passage evidently refers to the reception of one portion of the Israelites into favor after their rejection; in Peter, it refers mainly to those who had been Gentiles, and who had never been recognized as the people of God. The language of the prophet would exactly express his idea, and he therefore uses it without intending to say that this was its original application. See it explained in the notes on Romans 9:25.

Compare Ephesians 2:11 and following.

Which had not obtained mercy. That is, people who had been living unpardoned, having no knowledge of the way by which sinners might be forgiven, and no evidence that your sins were forgiven. They were then in the condition of the whole heathen world, and they had not then been acquainted with the glorious method by which God forgives iniquity.