When the meaning of certain verses, such as 1 Peter 2:9, is being discussed do you ever read the surrounding verses for context and do you ever take other scripture into consideration?
Of course.
Notice verse 10 here. That helps to determine who Peter was talking to here. It would make no sense for Peter to say that his fellow Israelites were never the people of God, but now they are.
Did you notice verse 1?
They were the people of God for a long time. It is the Gentiles who once were not a people, but now are the people of God along with Israelite believers.
The reference to "not a people" comes from Hosea, where the Lord declares that the Northern Ten Tribes are "not my people." I'll explain.
First, a review. At the schism of Shechem, the Northern Ten Tribes split off from Israel and formed their own government. These 10 tribes were called "The House of Israel." The remaining two tribes were called "The House of Judah."* During the time of Ahaz, God told the king that the Northern Ten Tribes would fail to attack Judah because the Assyrians were going to capture Israel and take them away into the nations, but God would not allow the Assyrians to attack Judah.
Due to this unfortunate situation, the Ten Tribes became the Jewish diaspora, living as aliens and foreigners in foreign lands and away from their kinsmen. Thus, Peter opens his letter, "To those who
reside as aliens, scattered throughout Pontus, Galatia, Cappadocia, Asia, and Bithynia, who are chosen 2 according to the foreknowledge of God the Father, by the sanctifying work of the Spirit, to obey Jesus Christ and be sprinkled with His blood: May grace and peace be yours in the fullest measure." In other words, Peter is talking to the sons and daughters born of the Ten Tribes who confess Jesus Christ. These folks, Peter says, were sanctified by the Spirit.
With this background under our belt, we can now understand the Lord word spoken through Hosea and his children about the Northern Ten Tribes who were once "a people" but were declared "not a people" by the Lord. One mode of Hosea's ministry was to name his children according to his prophetic word.
Hosea 1:
6 Then she conceived again and gave birth to a daughter. And the Lord said to him, “Name her Lo-ruhamah, for I will no longer have compassion on the
house of Israel, that I would ever forgive them. 7 But I will have compassion on the
house of Judah and deliver them by the Lord their God, and will not deliver them by bow, sword, battle, horses or horsemen.”
8 When she had weaned Lo-ruhamah, she conceived and gave birth to a son. And the Lord said, “Name him Lo-ammi, for you [the house of Israel] are not My people and I am not your God.”
Here we see that the Lord refers to the house of Israel as "not my people." And so Peter, addressing the diaspora Christians, refers to them as a holy nation and a people who were once "not a people". Both of these characteristics are true of the diaspora.
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* Note: the New Covenant Passage of Jeremiah refers to both of the houses by name: Jeremiah 31:31 Behold, days are coming,” declares the Lord, “when I will make a new covenant with the
house of Israel and with the
house of Judah,
Another thing to consider here is what Peter said in 1 Peter 2:5-7. He talks about them being "built into a spiritual house" which has Jesus as its "cornerstone". That reminds me of this passage where Paul was writing about Gentiles who had previously been "separate from Christ, excluded from citizenship in Israel and foreigners to the covenants of the promise, without hope and without God in the world" (Eph 2:12).
Yes. They are similar ideas. Peter is talking to members of the Ten Tribes who confess Christ, and Paul is talking about Gentiles who confessed Christ. Both of them have the adoption as sons because both of them have been sanctified by the Spirit.