Paul continues his argument that the current (for him) rejection of the descendants of Israel is not final. His argument is that the "gifts and calling of God are irrevocable (11:29)." Because God was forming a people for Godself through the Old Testament the people that were formed are not completely rejected. There will be a remnant (vs 5). God always preserves a remnant. He points out to his largely gentile audience in Rome that the gentiles have been "grafted" on to the holy olive branch of ancient Israel. We are like wild olive shoots that have been masterfully grafted onto to a much older olive tree. Because we are the grafted ones we should not brag about our new status in faith. Instead we should be grateful, humbly grateful, for what God has made possible. We stand "in awe" of Gods mercy (11:20)
The question that Paul wrestles with here is not one the modern world spends much time discussing. In Paul's time this was a critical question. If God has made a way for the gentles (the non Jews) does that mean that God has rejected the Jews from salvation? Paul emphatic answer is by no means. God will make a way for his chosen. How God does that is not our concern. Our concern is to spread the good news of the Gospel of Jesus Christ to every corner of the planet; to teach and tell all who will listen. Those we cannot reach we trust to our gracious and merciful God for God's ways are unsearchable and inscrutable (11:33).