When it comes to the new covenant promise in Jeremiah 31:31-40, there are four questions that we should ask about the text. The first question was, who is the New Covenant between? The second question is, What does the New Covenant include?
The new covenant is something new and Jeremiah explicitly stated that it is “not like the covenant” the LORD “made with their fathers” coming out of Egypt. This is not just an agreement with requirements and benefits, this is an agreement wherein God gives the additional benefit of causing Israel to fulfill the requirements for sake of receiving the other benefits. That is new, and significantly different.
Instead of stone tablets, “I will put my law within then, and I will write it on their hearts” (Jeremiah 31:33). Ezekiel described it as, “I will give you a new heart and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules” (Ezekiel 36:26-27). This is exactly what they could not do on their own. They could not give themselves new hearts. The theological label for this is regeneration, spiritual life, and the in-dwelling of the third Person of the Trinity. This is salvation.
The covenant includes forgiveness: “I will forgive their iniquity, and I will remember their sin no more” (Jeremiah 31:34). The covenant includes righteousness: “I will sprinkle clean water on you, and you shall be clean from all your uncleanness, and from all your idols I will cleanse you” (Ezekiel 36:25, 29, 33).
The fulfillment of these promises does not depend on Israel’s repentance, the promise is that God will grant Israel repentance. There is no condition for Israel to meet. The entire point of the new covenant is that God will do everything necessary to save Israel because they have not obeyed what He said previously.
These are not the only parts of the new covenant, however. Connected to this covenant is that “the city shall be rebuilt for the LORD” (Jeremiah 31:38), with specific place names mentioned (“from the tower of Hananel to the Corner Gate”) to leave no doubt that the LORD means Jerusalem, and even how big Jerusalem will be. In verse 40 God includes the valley and fields outside of the city that “shall be sacred to the LORD” (verse 40).
Ezekiel is just as specific. Immediately following the promise to “cause you to…be careful to obey my rules,” the LORD says, “You shall dwell in the land that I gave to your fathers” (verse 28). Then, “I will deliver you from all your uncleannesses. And I will summon the grain and make it abundant and lay no famine upon you” (verse 29, also verse 30). Then “says the LORD God: On the day that I cleanse you from all your iniquities, I will cause the cities to be inhabited, and the waste places shall be rebuilt. And the land that was desolate shall be tilled” (verse 33). “They will say, ‘This land that was desolate has become like the garden of Eden, and the waste and desolate and ruined cities are not fortified and inhabited’” (verse 35). The New Covenant promises new hearts that will desire to obey God’s laws as well as all the material/earthly blessings promised by the LORD to Abraham, Isaac, and Jacob: land, a city, fields, grain in abundance.
I have heard it argued that the “land of their fathers” just represents that God’s people—all believers both Jew and Gentiles—will inhabit the whole earth. It’s said that a Jew who heard the new covenant promises would have understood the promise of grain and of no famine as symbolic of having plenty, not has having anything specific to do with where such fruitfulness came from. It could be in China, or Alaska, and a believing Israelite would have been fine with that. But what makes us think God has changed His terms?