I take it you’ve never grafted a branch before.
The Jews were “broken off” due to their unbelief in Christ, but “if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again.” (Romans 11)
There would be no need to graff in the Jews AGAIN if they were never broken off. If the Jews can be God’s chosen and of the saved while SEPARATE and APART from Christ remaining in UNBELIEF of Christ then Christ’s death was in vain. A fleshly Jew outwardly must become a spiritual Jew inwardly (become a Christian) to be of God’s chosen (Romans 2:28-29)