St. Paul gives us a key to helping us get our cards laid out.
"I ask, then, has God rejected His people? By no means! For I myself am an Israelite, a descendant of Abraham, a member of the tribe of Benjamin. God has not rejected His people whom He foreknew. Do you not know what the Scripture says of Elijah, how he appeals to God against Israel? 'Lord, they have killed Your prophets, they have demolished Your altars, and I alone am left, and they seek my life.' But what is God's reply to him? 'I have kept for Myself seven thousand men who have not bowed the knee to Baal.' So too at the present time there is a remnant, chosen by grace. But if it is by grace, it is no longer on the basis of works; otherwise grace would no longer be grace." - Romans 11:1-6
The Apostle compares the remnant of faithful Israel who refused to bow to Baal with those who have embraced Jesus as Messiah and Lord. This is relevant, because when the Apostle talks about the inclusion of the Gentiles, he speaks of the Gentiles as being included in the commonwealth of Israel, as wild branches grafted onto the olive tree.
"Therefore remember that at one time you Gentiles in the flesh, called 'the uncircumcision' by what is called 'the circumcision', which is made in the flesh by hands--remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. For He Himself is our peace, who has made both one and has broken down in His flesh the dividing wall of hostility by abolishing the law of commandments expressed in ordinances, that He might create in Himself one new man in place of the two, so making peace, and might reconcile both to God in one body through the cross, thereby killing the hostility. And He came and preached peace to you who were far off and peace to those who are near. For through Him we both have access in one Spirit to the Father. So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus Himself being the cornerstone, in Whom the whole structure, being joined together, grows into a holy temple in the Lord. In Him you also are being built together into a dwelling place for God by the Spirit." - Ephesians 2:11-22
In the Messiah the Gentiles have been grafted in, brought in, and along with believing Jews are together one new people; there is neither Jew nor Greek. In Christ both Jew and Gentile are one. The Gentiles are now children of the same promises given to Israel, they are members of Israel, and Abraham is the common father of all who have faith in the Messiah. And in this way the promise made to Abraham is fulfilled, that he would be the father of many nations, through the promised Seed--that is, Jesus Christ (see Galatians 3:16).
What has happened to Israel then? Has it been replaced? Of course not. Israel has always been Israel. But Gentiles, formerly estranged, foreign, having no participation in the covenant and promises of Israel are now, through the Messiah, included and are full citizens along with the Jews. So that Jew and Gentile, together in the Messiah, are Israel. But even as it was in the days of Elijah, where there were those who were faithful to YHWH and those who were unfaithful; faithfulness to YHWH is in believing in the Messiah--in Jesus. Not that God has turned His back on the Jewish people; but that those who are in the Messiah remain faithful to YHWH; and every Gentile who formerly was an alien and a stranger has come to know YHWH and is now a member of His House, His Covenant People. It is not circumcision in the flesh that matters, but spiritual circumcision of the heart that matters--for the one who has been baptized has put off the old self and is made new in the Messiah (Galatians 3:21, Colossians 2:11-13).
For there is a New Covenant established in the blood of the Messiah, as it was in ancient times foretold by Jeremiah the Prophet,
"Behold the days are coming, declares YHWH, when I will make a new covenant with the house of Israel and the house of Judah, not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, My covenant that they broke, though I was as their husband, declares YHWH. For this is the covenant that I will made with the house of Israel after those days, declares YHWH: I will put My law within them, and I will write it on their hearts. And I will be their God, and they will be My people. And no longer shall each one teach his neighbor and each his brother, saying, 'Know YHWH,' for they shall all know Me, from the least of them to the greatest, declares YHWH. For I will forgive their iniquity, and I will remember their sin no more." - Jeremiah 31:31-34
The author of Hebrews adds this:
"In speaking of a new covenant, He makes the first one obsolete. And what is becoming obsolete and growing old is ready to vanish away." - Hebrews 8:13
The Old Covenant was established in order that it might serve to point to Messiah and His new and better Covenant. For Christ doesn't point to Moses, but Moses to Christ. It is Christ who is the spiritual rock in the wilderness (1 Corinthians 10:1-4). He brings in a new and better priesthood, one that isn't established on the basis of genealogy, but which precedes Aaron and Levi, in the order of Melchizedek. And He offers as Great High Priest the once-and-perfect Sacrifice of Himself which has atoned for the sins of the whole world.
