Hebrews 8:8 says plainly, the new covenant is made with the house of Israel and Judah...how could that be said to apply to the body of Christ. It could only be applied to the believing remnant of Israel, the church Paul persecuted, before there was a body of Christ.
I really don't know where to begin a response to you on that matter, because there is so much written about, but mainly in the Old Testament prophets. I can only offer this summary, but you'll have to do the leg work in the OT if you really want to understand this:
1. Solomon allowed his foreign wives to bring in their idols into Israel. God warned Israel against idol worship and what He would do if His people did that per Deut.4 & 28, i.e., scatter them among the Gentiles.
2. God rent the kingdom from Solomon's son Rehoboam, gave him 2 tribes at Jerusalem called the "house of Judah". God then gave Jeroboam, one of the tribe of Ephraim, as king over ten tribes in the north at Samaria, called the "house of Israel". Old Israel was split into two separate kingdoms.
3. King Jeroboam over the northern ten tribes then created false idol calf worship in the north to keep the ten tribes from going south to Jerusalem to side with Judah and Benjamin. He made common priests of the people, which prevented the Levites from doing their duties in the north, so they left and went south to side with the tribes of Judah and Benjamin. A small number of the ten tribes also refused Jeroboam's golden calf idols and went south to join with Judah. The tribes of Judah, Benjamin, and Levi then made up the "kingdom of Judah" or "house of Judah" per God's Word.
4. After many kings in both kingdoms, God got tired of the continued golden calf idols that the northern "kingdom of Israel" kept doing, so He brought the kings of Assyria (in stages) against the northern ten tribes and removed them all out of the land. Only the 3-tribed house of Judah was left in the land at Jerusalem/Judea, which called themselves Jews after the name Judah.
5. The kings of Assyria then took pagan peoples from five different cities in Babylon, each city with its own pagan idol worship, and planted those peoples in the northern lands of Israel where the ten tribes had been removed. These became known as the Samaritans. They were foreigners and the Jews in the south wouldn't have anything to do with them. This is why Christ's Apostles even questioned our Lord Jesus' behavior when He spoke with the Samaritan woman at the well.
6. The ten tribes became lost, to themselves and to the world. What God had warned in Deut.4 & 28 came to pass. However, OT prophecy involving the ten tribes of the "house of Israel" continued, even prophecies about them all the way to the end (Amos 9:9).
7. In Romans 9, Apostle Paul quoted from the Book of Hosea, showing a section of that as fulfillment about the Gospel going to the Gentiles. Paul was speaking to the Church in Romans which was to Gentiles. However, the Hosea Scripture he quoted was written specifically about the ten tribe northern kingdom of Israel.
Rom 9:24-26
24 Even us, whom he hath called, not of the Jews only, but also of the Gentiles?
25 As he saith also in Osee, "I will call them My people, which were not My people; and her beloved, which was not beloved.
26 And it shall come to pass, that in the place where it was said unto them, 'Ye are not My people'; there shall they be called the children of the living God."
KJV
You have to study the flow of Hosea 2 with what God is saying there about the ten tribes to grasp just 'how' Paul applied that above quote in Romans 9. I can't explain it any other way, so this may take a while...