Some have taken the fact that Gentiles have been grafted into Israel’s natural olive tree to mean that Gentiles are now actually Jews. They might try to argue the point by highlighting several verses in Romans (for instance in Romans 2 where Paul points out uncircumcised Gentiles living circumcised lives proving themselves to be more circumcised than Jews).>
But Paul makes a point to maintain the identity of the Gentile and the Jew in mostly every passage where he deals with the two. Let me unpack that a bit.
Romans chapter 3 Paul has no problem pointing out that both Jews and Gentiles stand on the same ground before God: deserving His wrath for all have sinned and come short of His glory. But before reaching that climax he makes a point to highlight that the Jews have benefited much in every way.
In fact, when Paul speaks about the Olive Tree he highlights that natural Israel is being set aside until the bit with the Gentiles is completed. After the Gentile number is filled then All Israel will be saved. He maintains their corporate identity in both cases right through the execution of God’s partial hardening.
Elect Israel is, in regards to the gospel, an enemy for the Gentile’s sake but concerning their election they are beloved for the sake of the patriarchs. The distinction is maintained.
Paul sees support throughout the prophets that the Deliver will come from Zion and He will turn away ungodliness from Jacob specifically: that is Israel. For this is God’s covenant with Israel when He promises to take away their sins—and His promise is irrevocable. God will do this at the very point when all of Israel realizes that they are not truly believers at which point God shows mercy.
In the end, therefore, a literal Israel has a promised future with a literal fulfillment based on God’s promises in His covenants…and His word must stand.