In this chapter Paul points to himself as an example of the fact that there is a remnant of Israel who has been saved, in this case Israel alludes to the majority who are Jewish from the ethnic point of view, including their recognized leadership, having Failed to get a proper relationship with God by their own efforts, they hardened.
Persistent and stubborn disbelief caused God's justice to fall upon them, in fact the unbelief of Israel opened the door of opportunity to the Gentiles.
God's purpose is that when unbelieving Jews see a large number of Gentiles turn to Christ, they will become jealous, repent, and come to faith.
Moreover, Paul here warns the Gentiles against boasting, pride and arrogance, on account of the fact that they are a wild olive tree ... grafted, because of Israel's unbelief.
The rejection of Israel is temporary, until all those who are going to be saved among the Gentiles deposit their trust in Christ, then salvation will come to a greater number of Jews, in the same way that has happened with other peoples through the centuries, a response of faith to the preaching of the gospel of Christ.