In the purest sense, the New Covenant mentioned in Jeremiah 31:31-34 is solely with Israel, but with a provision to include the rest of mankind (Gentiles). This is Paul’s argument in Romans 10-11.
Paul in Romans 15:8-13 quotes several OT scriptures to validate God’s plan to include the Gentiles in the salvation plan instituted by the New Covenant. This also validates the promise made to Abraham (Gen 12:3) that in him all families of the earth would be blessed (also described by Paul in Gal 3:8 as the “gospel”).
This idea that “salvation has come to the gentiles through the Jews” was reiterated by Paul in Romans 15:26-27, “26 For Macedonia and Achaia have been pleased to make some contribution for the poor among the saints at Jerusalem. 27 For they were pleased to do it, and indeed they owe it to them. For if the Gentiles have come to share in their spiritual blessings, they ought also to be of service to them in material blessings.”
So, the New Covenant is solely with Israel, yet there is a provision to include the Gentiles in Israel’s spiritual blessings & salvation.
So, to put it all together in a nutshell:
- God’s plan was always to save the entire human race (Rom 5:18-19).
- He chose one man Abraham through which to bring about this blessing to all families (Gen 12:3).
- Following one of Abraham’s lines (Abraham, Isaac, Jacob), God chose a nation unto himself a) with whom he made a temporary conditional covenant, giving them the law to set them apart, and also to give a picture of, through the sacrificial system, the final work that the Redeemer would accomplish on behalf of mankind; and b) through whom that Redeemer would arise.
- Seeing ahead, God also talked about a New Covenant to come that he would establish with Israel (Jer 31). He also talked about the inclusion of the Gentiles into Israel’s spiritual blessings (Deut 32:43; Ps 117:1; Is 11:10).
- Christ’s death was the initiation of this new covenant with Israel (Heb 8, 9).
- Since this new covenant makes the old one obsolete, the old is no longer in effect as a covenant, yet, the principle of “God being morally different from people” still applies. So, we can use the law as a means of exposing sin and our need for salvation (Rom 3:20; Gal 3:24).
- Once God’s plan for inclusion of the Gentiles is complete, He will again return his focus to Israel and fulfill all the promises made to them in this New Covenant (Rom 10-11).
God’s salvation plan truly is astounding. This is the reason for Paul’s doxology at the end of Romans 11.