I speak the truth in Christ—I am not lying, my conscience confirms it through the Holy Spirit—I have great sorrow and unceasing anguish in my heart. For I could wish that I myself were cursed and cut off from Christ for the sake of my people, those of my own race, the people of Israel. Theirs is the adoption to sonship; theirs the divine glory, the covenants, the receiving of the law, the temple worship and the promises. Theirs are the patriarchs, and from them is traced the human ancestry of the Messiah, who is God over all, forever praised! Amen. (Romans 9:1-5)

But, though given all the advantages of first right of refusal, many Jews refused the gift. Unfortunately, the promise was not an automatic to everyone; rather, it was that the Lord’s salvation [reckoning] was merely by way of [through Isaac], i.e., the Jews:

It is not as though God’s word had failed. For not all who are descended from Israel are Israel. Nor because they are his descendants are they all Abraham’s children. On the contrary, “It is through Isaac that your offspring will be reckoned.”(Romans 9:6-7)

In other words, it’s not lineage that makes one saved. It is the children of the promise – understood now, post-Jesus, to be all those God chooses to save through His plan of salvation: first through Abraham’s offspring [Isaac], then Isaac’s offspring [Jacob], not even the first-born [Esau], but the younger of the two brothers.

After all, it is God’s sovereign choice.

In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring. For this was how the promise was stated: “At the appointed time I will return, and Sarah will have a son.” (Romans 9:8-9)

Not only that, but Rebekah’s children were conceived at the same time by our father Isaac. Yet, before the twins were born or had done anything good or bad—in order that God’s purpose in election might stand: not by works but by him who calls—she was told, “The older will serve the younger.” Just as it is written: “Jacob I loved, but Esau I hated.” (Romans 9:10-13) 

By the way, the love/hate words above are not referencing the common emotions we think of today, but reference God’s sovereign choice. This quote above from Malachi 1:2-3 is referencing God’s favor of chosen Israel [Jacob] as opposed to non-chosen Edom or the Edomites [descendants of Esau].¹

What then shall we say? Is God unjust? Not at all! For he says to Moses,

It does not, therefore, depend on human desire or effort, but on God’s mercy. For Scripture says to Pharaoh: “I raised you up for this very purpose, that I might display my power in you and that my name might be proclaimed in all the earth.” Therefore God has mercy on whom he wants to have mercy, and he hardens whom he wants to harden. (Romans 9:14-18)

So one might complain that if God does what God wants, what power do I have as a mere man? So therefore, how can God even blame us for anything? Think again, Paul says, Who are you to dictate what God, the Master, does?

One of you will say to me: “Then why does God still blame us? For who is able to resist his will?” But who are you, a human being, to talk back to God? “Shall what is formed say to the one who formed it, ‘Why did you make me like this?’” Does not the potter have the right to make out of the same lump of clay some pottery for special purposes and some for common use? (Romans 9:19-21)

What if God, showing great patience, power, wrath, and mercy, displayed it all in playing out the riches of His glory for who He wills, both the Jews and the Gentiles? He is, after all, God of all things:

What if God, although choosing to show his wrath and make his power known, bore with great patience the objects of his wrath—prepared for destruction? What if he did this to make the riches of his glory known to the objects of his mercy, whom he prepared in advance for glory—even us, whom he also called, not only from the Jews but also from the Gentiles?

As he says in Hosea:

It is just as Isaiah said previously:

Israel’s Unbelief
So, the Gentiles received righteousness, available by their own choice of faith. The Jews tried to keep the law to pursue righteousness, but missed the goal by way of works rather than faith.

They stumbled over the stumbling stone, the Rock of Ages, Jesus, the Messiah, savior of the whole world.

What then shall we say? That the Gentiles, who did not pursue righteousness, have obtained it, a righteousness that is by faith; but the people of Israel, who pursued the law as the way of righteousness, have not attained their goal. Why not? Because they pursued it not by faith but as if it were by works. They stumbled over the stumbling stone. As it is written:

Who are God’s People?
So, who are God’s people? Any who believe by faith in God’s redemptive gift, Jesus Christ. This could be steadfast Jews who move from Judaism to Christianity through faith, or any Gentiles who become Christ-followers as well through faith.

This recalibration of God’s chosen people hasn’t changed in 2,000 years since the death and resurrection of Christ.

Do you recognize how God used human history to reconcile mankind back to Him?
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In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring. – Romans 9:8

¹ Note: for a good explanation of this confusing hatred for Esau, see Why Did God Love Jacob and Hate Esau? in GotQuestions.org.