Romans 9 is an interesting and often difficult passage. I’m going to very briefly sketch the way I am inclined to read it, particularly to note the way I don’t think it supports the Calvinistic doctrine of unconditional election, but rather undermines it.
Contextual setup: Paul has been defending his Gospel of justification by the faithfulness of Jesus the Messiah and our union with Him instead of justification by observing the Torah. This naturally leads to the question: what has happened to the Jews who have rejected Jesus? They seem cut off from the promise. What does this mean for God and His faithfulness to the covenant?
First, Paul explains that he really does care. He loves his people, and he wants them to be saved. They’ve gone through so much with God already. But that’s just it. God hasn’t been unfaithful. God has always worked this way, limiting and redrawing the limits of the people of God by His sovereign choice. It began with Abraham, but being a descendant of Abraham was not enough to inherit the promise later, when God chose to narrow election to Isaac’s line. Was this because Isaac was more worthy than Ishmael? No, for in the next generation God narrowed election through Jacob rather than Esau before either had been born.
Did this make God unjust? Certainly Paul knew his audience would agree God had the right to do this. He has mercy as He wills, and sometimes He actually hardens people in order to accomplish His purposes in election. When He chose Moses and Israel under his leadership to finally come out as His people, He hardened the (already wicked) Pharaoh so that His salvation for Israel might be all the more fully displayed, and indeed that His name would reach the ends of the earth. (Paul gives no indiciation here of a reprobation to damnation for Pharaoh. All that has been mentioned appears to be a particular act at a redemptive-historical moment for Pharaoh to rebel.)
This is entirely justified. God is allowed to redraw election and harden its enemies whenever and however He pleases, despite man’s objections. He can freely form and reform Israel as a potter does clay and judge the vessels which are cut off by this reshaping. Therefore now Israel is shaped around, her election narrowed to, Christ rather than merely Abraham and circumcision or Moses and the Torah. There is no injustice in this, even if a great deal of clay is now set aside for damnation.
But ironically, Paul points out that this narrowing also expands election. By making Christ the new head of Israel’s election (as God did before with Isaac and Jacob) and hardening many of the Jews to reject Him, the door has swung wide open for Gentiles to inherit the promise and join the people of God. Now even the Gentiles can enter the chosen people through union with Christ by faith, the barriers of the Torah and Abrahamic descent overcome.