Jeremiah 31:31-34
“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, 32 not like the covenant which I made with their fathers in the day I took them by the hand to bring them out of the land of Egypt, My covenant which they broke, although I was a husband to them,” declares the LORD. 33 “But this is the covenant which I will make with the house of Israel after those days,” declares the LORD, “I will put My law within them and on their heart I will write it; and I will be their God, and they shall be My people. 34 They will not teach again, each man his neighbor and each man his brother, saying, ‘Know the LORD,’ for they will all know Me, from the least of them to the greatest of them,” declares the LORD, “for I will forgive their iniquity, and their sin I will remember no more.”
Given that Jeremiah states and the author of Hebrews quotes that this covenant is with the houses of Israel and Judah, and that the New Covenant is never applied to Gentiles in any passage of the New Testament (as far as I can think of off the top of my head), is the New Covenant only for Israel? The only passage I can think of right now where Gentiles and the New Covenant have any interaction is 1 Corinthians 11, when Paul instructs Gentiles on the proper observance of the Lord's Supper. Also, Paul is clear in Galatians 3 that Gentiles are reconciled to God on the basis of God's promises to Abraham in Genesis 12 and 22, not through the Mosaic covenant.
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Rom 9:4: They are Israelites, and to them belong the sonship, the glory, the covenants, the giving of the law, the worship, and the promises;
5: to them belong the patriarchs, and of their race, according to the flesh, is the Christ. God who is over all be blessed for ever. Amen.
6: But it is not as though the word of God had failed. For not all who are descended from Israel belong to Israel,
: Notice in verse 4 that to the Israelites belong the covenants. Paul states that not all who are physically born as Israelites belong to Israel. To put it another way, true Israelites are those who cling to Israel's Messiah by faith.
Also in verse 4: The sonship belongs to Israel. To be a son of God, you must be an Israelite. That is to say, Israel is God's family. Tie this together with verses 24 - 26 in which gentiles are referred to by God (quoted from Hosea) as "my people" and "sons of the living God."]
24: ... not from the Jews only but also from the Gentiles?
25: As indeed he says in Hosea,
"Those who were not my people
I will call `my people,'
and her who was not beloved
I will call `my beloved.'"
26: "And in the very place where it was said to them, `You are not my
people,' they will be called `sons of the living God.'"
27: And Isaiah cries out concerning Israel: "Though the number of the sons of Israel be as the sand of the sea, only a remnant of them will be saved;
30: What shall we say, then? That Gentiles who did not pursue righteousness have attained it, that is, righteousness through faith;
31: but that Israel who pursued the righteousness which is based on law did not succeed in fulfilling that law.
32: Why? Because they did not pursue it through faith, but as if it were based on works. They have stumbled over the stumbling stone,
33: as it is written,
"Behold, I am laying in Zion a stone that will make men stumble,
a rock that will make them fall;
and he who believes in him will not be put to shame."