God made several covenants. He made one with Noah, one Abraham, one with Isaac and one with Jacob. He promised to guide and care for them and protect them. Those were individual covenants. But when Israel became a nation under Moses, God made a Covenant with Israel. It is called the Covenant of Law. Because of sin, Israel was unable to keep the Law Covenant so God in his mercy, devised a New Covenant where sin would not be a deciding factor.
Over the course of the Centuries Israel split into two houses; in the North Israel and in the South Judah.
Jer 31:31 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.
Hebrews 8:8 quotes this exactly and adds
Heb 8:13 In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.
Ezek.36:26 I will give you a new heart and put a new spirit in you. I will remove your stubborn hearts and give you obedient hearts.
This a promise of the New Covenant we will get new loving, obedient hearts and we will receive a new Spirit –the Holy Spirit to help us.
Every covenant had a mediator and was sprinkled with blood. Hebrews says
Heb 12:24 Jesus is the mediator of the new covenant, and to the blood of sprinkling,
At the Last supper Jesus spoke of pouring out his blood for the New Covenant.
Luk 22:20 In the same way, he gave them the cup after the supper, saying, "This cup is God's new covenant sealed with my blood, which is poured out for you.
Mat 26:28 "this is my blood, which seals God's covenant, my blood poured out for many for the forgiveness of sins.
Now the question is if this covenant is only with the houses of Israel and Judah how can gentiles be part of it.
Eph 2:8 God saved you through faith as an act of kindness. You had nothing to do with it. Being saved is a gift from God. It's not the result of anything you've done, so no one can brag about it. God has made us what we are. He has created us in Christ Jesus to live lives filled with good works that he has prepared for us to do. Remember that once you were not Jewish physically. Those who called themselves "the circumcised" because of what they had done to their bodies called you "the uncircumcised." Also, at that time you were without Christ. You were excluded from the Commonwealth of Israel, and the pledges God made in his promise were foreign to you. You had no hope and were in the world without God. But now through Christ Jesus you, who were once far away, have been brought near by the blood of Christ. So he is our peace. In his body he has made Jewish and non-Jewish people one by breaking down the wall of hostility that kept them apart. He brought an end to the commandments and demands found in Moses' Teachings so that he could take Jewish and non-Jewish people and create one new humanity in himself. So he made peace. He also brought them back to God in one body by his cross, on which he killed the hostility. He came with the Good News of peace for you who were far away and for those who were near. So Jewish and non-Jewish people can go to the Father in one Spirit. That is why you are no longer foreigners and outsiders but citizens together with God's people and members of God's family. Eph 2:19
So by faith in Jesus you become a citizen of the Commonwealth of Israel, a member of God’s family.
In Romans 11 Paul describes Israel as an Olive tree that has holy roots and branches. He says most of the branches were broken off because they did not believe in Jesus, and the gentiles were from a wild Olive tree and they were broken from the wild tree and grafted onto the good Olive tree so that they are now part of Israel.
Rom 11:13 I am speaking now to you Gentiles…
Rom 11:16 If the roots of a tree are holy, so are the branches also.
Rom 11:17 Some of the branches (Jews) of the cultivated olive tree have been broken off, and a branch of a wild olive tree has been joined to it. You Gentiles are like that wild olive tree, and now you share the strong spiritual life of the Jews.
Rom 11:18 So then, you must not despise those who were broken off like branches. How can you be proud? You are just a branch; you don't support the roots---the roots support you.
Rom 11:19 But you will say, "Yes, but the branches were broken off to make room for me."
Rom 11:20 That is true. They were broken off because they did not believe, while you remain in place because you do believe.
So here you have it, believers do not replace Israel they become Israelis. They are the children of Abraham and today they number 2 billion, so the promise that Abraham’s seed would be like the sand of the seashores is coming to pass.
“It is not the man who is a Jew on the outside, whose circumcision is a physical thing. Rather the real Jew is the person who is a Jew on the inside, that is, whose heart has been circumcised, and this is the work of God's Spirit. (Rom 2:29)
So Christians whose hearts are circumcised by faith in Jesus are true Jews.
Gal 3:7 Know ye therefore that they which are of faith, the same are the children of Abraham.
1 Pet 2:9 However, you are the chosen people, a royal priesthood, a holy nation, people who belong to God. You were chosen to tell about the excellent qualities of God, who called you out of darkness into his marvelous light.
Conclusion
When you believe in Jesus, you become a child of Abraham, and an Israeli Jew, a fellow citizen in the Commonwealth of Israel, and a lifetime member of the Everlasting New Covenant. It is a gift sealed in blood. God never takes back any gifts.
Isa 54:10 For the mountains shall depart, and the hills be removed; but my kindness shall not depart from thee, neither shall the covenant of my peace be removed, saith the LORD that hath mercy on thee.
Eze 37:26 Moreover I will make a covenant of peace with them; it shall be an everlasting covenant with them: and I will place them, and multiply them, and will set my sanctuary in the midst of them for evermore.
Mal 2:5 My covenant was with him of life and peace