Jeremiah 31:37 This is what the Lord says: “Only if the heavens above can be measured and the foundations of the earth below be searched out will I reject all the descendants of Israel because of all they have done,” declares the Lord.
This is speaking of the rebellions of Israel, and thus not specifically of believers, and thus this is referring to the nation per se, ethnic Israel.
This prophecy doesn’t have to refer to every single Israelite, but it does refer to “the descendants of Israel,” I shouldn’t have said the nation per se, but ethnic Israelites, they will always exist, Jewish people, see verses 35-37, and Jer. 33:24-26. They will not be rejected “because of all they have done,” referring to historic incidents, and thus referring to the groups associated with these incidents, and thus meaning a group different than the NT church.
Here is another promise that is specific to Israel:
Romans 2:9-10 There will be trouble and distress for every human being who does evil: first for the Jew, then for the Gentile; but glory, honor and peace for everyone who does good: first for the Jew, then for the Gentile.
Jewish people are first in line, for glory or trouble, depending on their deeds.
Certainly everybody is treated equally in judgment, but Jewish people are first in the order. If we read “there is no longer Jew or Greek” (Gal. 3:28), Paul also says there is no longer male and female! But that doesn’t mean everyone is now the same gender.
And the nation of Israel will be the head of all nations:
Isaiah 60:12 For the nation or kingdom that will not serve you will perish; it will be utterly ruined.
And this refers to the nation, not to all believers:
Isaiah 60:21 Then will all your people be righteous and they will possess the land forever.
If this was believers, then of course they would all be righteous, this would not need stating! And also, possessing the land refers to geography, to the land of Israel, their inheritance, which was specifically given to Jewish people.
And though in one sense believers are called Israel, the sense of “Israel” as “the people descended from Israel” is also valid in the NT:
Romans 11:1 I am an Israelite myself, a descendant of Abraham, from the tribe of Benjamin.
And though the kingdom was given to others, the kingdom can be given back!
Acts 1:6 So when they met together, they asked him, “Lord, are you at this time going to restore the kingdom to Israel?”
And Jesus said “You are not to know the time,” not “no, this will never happen,” so the Christian church has not replaced Israel, there are still specific promises for them, and roles for them, and the earthly kingdom will be theirs, too.