Let us look at this passage from the OT to find out the truth because this prophecy foretells of a time in the future when all Israel will be saved:
"Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah: Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was an husband unto them, saith the LORD: But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more" (Jer.31:31-34).
First, we can understand that those who will belong to the houses of Israel and Judah will be the physical descendants of Abraham, Isaac and Jacob. That is because their "fathers" were the ones who broke the Lord's covenant and those people were the physical descendants of Jacob. Therefore, since their "fathers" are the physical descendants of Jacob then that must mean that they are also the physical descendants of Jacob.
Next, we can understand that all of the physical descendants of Jacob which will belong to both houses will know the Lord,
"from the least of them unto the greatest of them," and they will all have their sins forgiven.
Paul certainly understood that truth when he wrote this:
"For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in. And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob: For this is my covenant unto them, when I shall take away their sins" (Ro.11:25-27).
The "Israel" in these verses is contrasted with the "Gentiles" so this has nothing to do with the Body of Christ.