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The greatest stumbling block to many Jews who heard Paul's message was that his gospel excluded from God's kingdom unbelieving, yet "law-keeping," circumcised Jews, while it welcomed believing Gentile sinners! To Jews who took pride in their heritage, lineage, law, or circumcision, considering themselves favored above Gentiles, Paul's message was an insult. So in this chapter, Paul helps Jews see that God can choose whomever He wants and reject whomever He wants, regardless of what anyone thinks! Moreover, God has historically demonstrated that He doesn't make His selections of people based on those things that most Jews were trusting in to make them right before God, such as physical lineage, birth privileges, or even personal holiness.
In regard to physical lineage, Paul reminds his readers of what they certainly already knew, that although God chose Abraham for a special blessing, He did not choose all of Abraham's descendants. Moreover, it was Isaac the second-born, not Ishmael the first-born, who was surprisingly chosen to inherit the blessing. (And Paul cannot resist pointing out that Ishmael was a product of Abraham's works, while Isaac was a product of Abraham's faith---an analogy that teaches about salvation.)
Moreover, God surprisingly chose Jacob, not Esau, to next inherit the blessing, and His choice was made before they were born, so Jacob's blessing was not based on his works. Knowing this, how can any Jew object to God choosing to save Gentiles without regard to their works? Their forefather and namesake, Israel, was chosen by God without regard to his works!
May I point out that this chapter doesn't teach that God chooses some individuals for salvation and (thus by default) chooses other individuals for damnation. Only those who rip verses from their context within this chapter and the entire book can come to such a conclusion. This chapter, from beginning to end, is about Jews and Gentiles as groups of people, and God's choice to offer mercy. Additionally, God's choices of Abraham, Isaac and Jacob were not choices regarding their salvation. That was not Paul's point.
In the strongest terms, Paul reminds his readers that God is never unjust (9:14). So when it appears to us that God is unjust, it shows we have the wrong perspective. For example, God's choices of Abraham, Isaac and Jacob may have appeared to be unjust favoritism, but it was actually an expression of God's mercy to the whole world, as they were chosen to carry the seed that would bring blessing to everyone.
I might add that had God chosen to save only descendants of Israel, that would make Him unjust without argument. And if He sovereignly grants salvation to some and not others, as Calvinists claim, that would also make Him unjust without argument. If He is going to remain fair and just and mercifully offer salvation to any, He must offer it to all. Moreover, it is perfectly just for Him to withhold His mercy from those who spurn it, and punish them, as He did Pharaoh. Clearly, from reading the story of the Exodus, God showed mercy to Pharaoh, but Pharaoh hardened his heart, and God's mercy decreased with each additional judgment, to the point when God actively hardened Pharaoh's heart as a just punishment. "God has mercy on whom He desires, and He hardens whom He desires" (9:18), but whichever He does, He does it justly, not arbitrarily!
Praise God that, as we will read in just two chapters, "God has shut up all in disobedience so that He may show mercy to all" (11:32). The first "all" in that sentence and the second "all" both mean "all"---all Jews and all Gentiles.
Yes, one can remove from its context Paul's potter and clay analogy and make it appear that Paul is saying that one's salvation is entirely up to God, the potter, and has nothing to do with us, the clay. But in context, Paul can only be teaching that God, the potter, can save believing Gentiles and not save unbelieving Jews, both from the clay of humanity.