In Romans 11, Paul finally brings to a close the argument that he has been making throughout the entire letter concerning God's faithfulness in the form of salvation for Jews and Gentiles through Jesus. (Romans 12 -16 is not so much a part of this arument specifically as it is the ethical outworkings of this salvation in the life of the community at Rome.) Paul begins to wrap up his argument by comparing Israel in his own day to Israel in the days of Elijah. The prophet Elijah called upon God concerning Israel's extensive corruption and idolatry. It seemed that God had forsaken Israel and even Elijah himself. However, God assured Elijah that there was a remnant of 7,000 people who had not bowed the knee to the false God Baal. Paul concludes that the same must be true in Israel in his day. Although he, like Elijah, can not see the number of his fellow countrymen being saved that he might have expected, Paul holds out hope that there is a remnant of Israel that will come to be a part of the true Israel under Messiah Jesus. As a result, Paul does not believe that Israel's state of faithlessness and disobedience will persist. Instead, he concludes that eventually the salvation of the Gentiles will arouse jealousy in the Jewish people. That is, as the Gentiles enjoy the benefits of the gospel, the Jewish people will see that they are missing out on the very treasure that was supposed to be theirs as God's covenant people and as a result, will accept Jesus as the Messiah.
Until that time (it is not clear to me exactly when Paul expected this jealousy to begin to be aroused in the Jewish people) Paul wanted the Gentiles in his audience to know that they had no reason to boast in their acceptance into the kingdom. It may have very well been tempting for the Gentiles to consider themselves as better than the Jewish people since God was now working in them so abundantly. But Paul has been combating that notion for several chapters now and he is not about to let up at this late stage in the game. He reminds the Gentiles that the old branches (unfaithful Israelites) of the olive tree (true Israel) have only been broken off because of their unbelief/unfaithfulness. Gentiles, the wild and uncultivated shoots that they are, have only been grafted into the olive tree because of their faith/faithfulness. Therefore, Paul warns that if these Gentiles are unfaithful, they will just as quickly be broken off so Israelites who have become faithful might be grafted in again.
In speaking this way about the relationship between Jews and Gentiles, Paul is not only outlining the kind of humility that should accompany all Christian faith or even just wrestling with the problem of Israel that has controlled this argument for much of the letter. It is very likely that he is also addressing the specific historical setting of the church at Rome as well. In 49 A.D Emperor Claudius had expelled the Jewish leadership from the the imperial city. It is historically plausible that Paul is writing this letter at about the same time that this same Jewish leadership is making its way back into the city of Rome and beginning to estblish itself there once again. As a result, there is a good chance that there was some degree of conflict between those Jews who had been important leaders in the Roman church before they were expelled and the Gentiles who had become leaders in their absence. Therefore, when Paul reminds the Gentile Christians in Rome that they have been grafted into this tree of salvation, it is likely that Paul is ushering a call of humility, acceptance, and peace to the Gentile leadership toward the Jewish Christians who are returning to the city. Neither Jew or Gentile has earned their place on this life giving family tree and neither of them are immune to being broken off of it simply because they have made it in. Therefore, it is imperative that both groups accept each other with the same grace that has been extended to them by their common Lord, Jesus Christ. (The gospel reading (Matthew 15:21-28) which the lectionary editors have paired with this reading in Romans is a vivid illustration of the kind of attitude that Paul is calling for from the Gentile Christians in Rome as they recognize the enormity of the grace that allows them to take part in the salvation that has come through the Jewish people.)
This same call to humility continues to be important for us in the Church today as well. On the one hand, it continues to be important in how the Church thinks of its relationship to Israel. It is a reminder that we should be outsiders in this throughly Jewish story of salvation and yet God has chosen to include us a part of the family. On the other hand, I think this same principle can be expanded to address the attitude of regular church-goers toward those who do not often attend church today. Sometimes we can feel as though church attendance or involvement means that we are automatically "in" but Paul reminds us in this same passage the only reason we are in is because of God's grace and our continued trust, obedience, and faithfulness. If we become unfaithful, we too, will be broken off so that another can be grafted in. We must not allow our church attendance or involvement to lead to some kind of spiritual pride or arrogance. Instead, we must continually embody the humilty and compassion of Christ toward those who have not yet been grafted in if we are to truly remain a part of the tree ourselves.