No, God Is Forming One New Man in Place of Two By Grafting Gentiles Into His Covenant People, Israel
The belief that the church has wholly replaced Israel as God’s covenant people is known as replacement theology. Replacement theology started early in church history, as early as the late first/early second century. While many first-century Jewish people recognized that Jesus was their long-awaited Messiah, sadly, many did not. As a result, a split began to occur within Judaism between the followers of Jesus (known in the first century as the followers of “the way”) and those who flatly rejected the notion that Jesus was the Messiah. Many of the politically connected, high-ranking religious leaders within Judaism felt the need to protect their religion against this perceived heresy and did so by persecuting the followers of Jesus and inciting the Roman government against them.
Instead of responding the way Jesus would have wanted, within 100 years of His life, the church – now composed of thousands if not millions of gentiles – began to develop a certain hard-heartedness toward the people of Israel. It was not long before church fathers like Ignatius, Justin Martyr, Tertullian, Irenaeus, and Augustine began stripping Christianity of its Jewish heritage, forgetting that Christianity was, quite literally, birthed out of Judaism and God’s covenant with Israel.
Sadly, this error was passed on through the centuries to the Protestant Reformers (some of whom were nauseatingly antisemitic) and right into the modern church. While the entire New Testament is clear that Gentiles are not required to become Jewish to be saved, what God has done in Jesus is obliterate any possible modicum of division that could separate the two. He has quite literally “torn down the dividing wall of hostility between us” and formed, in Jesus, “one new man in place of two, so making peace” (Ephesians 2:14-15). In other words, Jews can be Jews, Gentiles can be Gentiles, and we all, together, constitute the Body of Christ. But still, many people misconstrue this fact into thinking that God’s covenant with Israel has been voided and Christianity has no connection to the Jewish people. This is not the case.
The Bible never, ever says that the church has replaced Israel as the covenant people of God. The fact of that matter is that God, from the very beginning, had a plan in place that would bring salvation to the entire world. It started when He made a covenant with one man, Abraham, and told that man: “in you, and in your offspring, all the nations of the world will be blessed” (Genesis 22:18). From here, God rose up a nation through whom He would one day bring the Messiah, who is both the fulfillment of God’s promises to Israel and the savior of the whole world. In bringing Jesus the Messiah through the nation of Israel, God has not voided His covenant with Israel or replaced Israel with the church; He has found a way to bring all those who believe in Jesus – both Jew AND Gentile alike – into one new covenant. Which, by the way, is still with the house of Israel. How has he done this? By “grafting” gentile believers into Israel.
While the Bible does not use “replacement language” when discussing Israel and the church, it does use the imagery of an olive tree. According to Romans 11, Israel is like a cultivated olive tree that now contains both natural branches (ethnic Jews who believe in Jesus) and “wild olive shoots” that have been “grafted in” to the tree (Gentile believers in Jesus) (Romans 11:17). The church has not replaced Israel. Rather, Gentile believers have been grafted into Israel.
Along with this, it’s important to note that gentile Christians, those wild olive shoots which have been grafted into God’s cultivated olive tree through faith in Jesus, have been given a warning: “don’t be arrogant toward the natural branches” (Romans 11:18). What do we make of this? It is God’s way of cautioning us that even though there are currently ethnic Jews (natural branches) who have been “cut off” from the tree through unbelief, we should not think that we are somehow better than them. For any Jew who comes to faith in Jesus, the Bible promises that they will be “grafted in again,” and will once again be restored as a natural branch on God’s cultivated olive tree (Romans 11:23). In other words, don’t be arrogant toward Jewish people, even those who have not yet come to faith in Jesus. Instead, be thankful that God, through His covenant with the people of Israel, has brought salvation to the whole world, even to your very life!
So, has God replaced Israel with the church? Absolutely NOT! He has used Israel, and His covenant with those chosen people, to bring salvation to the whole world. He did this via His Son, Jesus: the Messiah promised to Israel, the Head of the church, AND the Savior of all people. God has not canceled His covenant with Israel. He has replaced it with a new, expanded version. This new covenant with Israel covers any and all people, Jew or Gentile, who put their faith in Jesus as Lord and Savior. He invites us all in and promises that, even though we gentiles are ”wild olive shoots,” we too can be “grafted in” to become full members of His cultivated olive tree.
 Or, supersessionism  In the earliest days of the faith, the church was exclusively Jewish. Jesus' 12 disciples were all Jewish, and Jesus is, first and foremost, the Messiah promised to Israel. Separating Christianity from its Jewish heritage/foundation is like separating a bone from its marrow – it loses a vital part of its substance.  Jeremiah 31:31-33 makes this clear.  Romans 11:11-24  Ibid.