In Romans 9, Paul returns to the question which really drives the argument of Romans; what about Israel? Paul has just concluded chapter 8 with a magnificent, even poetic, description of the amazing love of God in Christ Jesus. Paul says that it is a love so strong and so deep that there is nothing in this world which can separate us from it. But there is a problem with that, a deep seated problem that runs to the very core of Paul's identity. Despite Paul's belief in the power of God's love, he also knows that there is a very important group of people who have become separated from God's will for them, namely, God's chosen people, Israel.
Paul knows simply by vitrue of observing the many churches that he founded that gentiles were coming to proclaim Jesus as Lord and Messiah in significant numbers while most Jews still rejected him. How could this be? Paul was, of course, thrilled that so many gentiles had been lead into the kingdom but he wondered how this could happen while God's own chosen people, those of the covenant, those who worshipped in the temple, those from whom Christ descended with regard to the flesh were unable to see the new way in which God was moving. Did this mean that God had failed to keep his promise to Israel? Since there was now a new covenant people, did that mean that the old covenant people had been completely abandoned? Had God simply given up on Israel and started over? Paul refuses to accept that as a possibility. He says specifically in v.6 "it is not as though the word of God had failed". So then, if God's promises to Israel have not failed, why is it that Israel is not responding to the fulfillment of those promises in Jesus?
Paul will take all of Romans 9-11 to attempt to answer that question and even at the end of chapter 11 he will say that the wisdom of God is unseachable and his ways unfathomable (11:33). However, Paul begins to answer it here in verses 1-13 by reminding his audience of what it actually means to be Israel. It does not mean simply being a fleshly descendant of Abraham. If being Israel were just about blood line then Ishmael would have been considered just as much a son of the covenant as Isaac but he was not. If being Israel were just about physical heritage then Esau would have been the one through whom Israel's story ran and through whom God's promises were fulfilled since he was the firstborn but it did not and they were not. Instead, God said before Jacob and Esau were born that the older would serve the younger, the exact opposite of the traditional rules of inheritance.
Paul argues in the rest of the chapter that all of this means that being Israel is a matter of God's choice rather than a matter of physical descendency from Abraham. The story of Israel itself shows that God was never limited by ethnicity. God has always had the ability to choose how he would fulfill his promises to Abraham. This does not render Israel obselete or unimportant in God's plan (as Paul's passion in 9:1-3 should make clear) but it does mean that Israel must recognize that the fulfillment of God's promises is not dependent on their ethnicity. Instead, God has chosen to fulfill his promises to Israel through Jesus Christ. God continues to be faithful to Israel although it is not in the way that Israel might have expected.