Paul completes his three-chapters-long exploration of the fate of Israel in Romans 11. He concludes that her estrangement from God is only temporary, serving the purpose of allowing the Gentiles to enter the kingdom while Israel is hardened.
Paul uses the analogy of "broken branches" that have been cut off from an olive tree to allow "wild" branches (Gentiles) to be grafted in. Because of this, the Gentile Christians should not boast - they stand only on the basis of their faith. And if wild branches can be grafted in to the olive tree, how much more readily will the natural "cultivated" branches be able to be grafted back in one day? And that is what Paul is counting on as he is assured that Israel's obstinance is limited and that one day God's people will be empowered to return to Him (vs. 26-27).
If all of this strikes you as a great mystery, you are not alone. Even Paul, who wrestled with these issues mightily, offers this assessment: "How unsearchable are his judgments and how inscrutable his ways!" (vs. 33)