To every Jew/Israelite who claims the New Covenant is for Israel alone as per Jeremiah 31, a Christian response…

Jesus Christ – Yeshua initiated the New Covenant in His blood at Golgotha (Matthew 26:27-28; Luke 22:20). Israel was to be the proselytizer of the New Covenant of Grace and fulfill Yahweh’s promise to Abraham as through Abraham’s seed would come One who is a blessing to every family of the Earth (Genesis 12:3) and that Blessing, Messiah Yeshua, came specifically to the lost sheep of Israel (Matthew 15:24) to ordain and proselytize them in the Gospel as Israel would assume responsibility for the Great Commission (Matthew 28) but Israel rejected Messiah, they murdered Him upon a tree and demanded that His blood be upon them and their children (Matthew 27:25); therefore, the Gospel was taken from Israel and given to the Gentiles until the fullness of the Gentiles has entered the Kingdom (Acts 13:46; Romans 11:25); also, for this reason, only a remnant of Israel’s children will escape the “second death” in Hell due to unbelief as Israel continues to pursue self-righteousness through law-keeping while denying the imputation of Messiah Yeshua’s righteousness by faith in Him as Messiah (Isaiah 10:22; Romans 9:27; Romans 10).

YES, Jeremiah prophesied of a New Covenant through Israel but Yahweh’s covenants with Israel have always been predicated upon national obedience (Deuteronomy 28); therefore, Yahweh’s eschatological plan for Time and Eternity will NOT be thwarted or stymied by the unfaithfulness of Israel’s children; therefore, Yahweh honored Israel’s free will rejection of Messiah and the New Covenant Gospel and subsequently turned to the Gentiles to honor the Gospel via the Great Commission.