Here's what one Bible scholar has said:
The Priority of God’s Call 9:6b–13
The thesis is supported by an argument that makes a distinction within Israel. God’s word has not failed because not all those descended from Israel are Israel. Paul distinguishes between ethnic Israel and true Israel on the basis of the Abraham story. Not all of Abraham’s children are seed, as God’s promise did not mean the “salvation” of all his descendants. The scriptural proof is Genesis 21:12, in Isaac shall your seed be called. Paul’s point here is the reverse of 4:13–18, where he argued that Abraham’s seed was more extensive than his physical descendants. But the central point is similar. The true children of Abraham are not defined by physical descent or ethnic origin.
Verses 8–13 interpret and support the argument of vv. 6–7. Verse 8 offers a commentary on the Genesis citation in v. 7b. The commentary makes a negative and a positive statement. First, the children of the flesh (e.g., Ishmael and his descendants) are not the children of God. The positive assertion is that the children of promise (e.g., Isaac and his descendants) are reckoned as seed. The commentary is supported with another Scripture citation, Genesis 18:10. The son born to Sarah is a gift of divine promise and initiative, I will come.
Paul’s first point is that from the beginning God has made a differentiation within Israel:
the ones not out of Israel
the children of the flesh
the children of God
the children of promise
God has a commitment only to the second group. While many Jewish contemporaries linked descent from Abraham with covenant salvation, some distinguished a “true Israel” from the larger group (CD 4:2–12; 4QFlor. 1:14–19; 1 En. 1:8–9). Paul uses the distinction to argue that God’s salvation and blessing is based solely on God’s choice and promise.
Paul adds another scriptural story for support. Ishmael and Isaac were the children of two different mothers and two different sex acts. But Rebekah got pregnant with twins through one sex act with one man (Gen. 25:21). And even here God made a choice before they were born, before they could do any works that deserved being selected.
Verse 11b offers the second commentary. The choice of Jacob over Esau is interpreted with a purpose clause, in order that the freedom of election in God’s purpose might remain not out of works but out of calling. The commentary is supported by two scriptural citations, Genesis 25:23 and Malachi 1:2–3, representing the Law and the Prophets. The first speaks of the two children as two nations. The second was interpreted in contemporary Judaism to say God hated Esau because of his “bad deeds.” Paul rejects that interpretation by using the text to assert that God made the choice before birth, and thus before any performance of deeds.
Several things are noteworthy about Paul’s first argument. The distinction between ethnic Israel and true Israel is based on the principle of selection. God made choices from the beginning, as illustrated in the history of the patriarchs. In seven verses Paul has introduced the central patriarchs of Israel’s story—Abraham, Isaac, and Jacob—to ground divine election solely in the call of God, not on people’s deeds. This call theme is repeated throughout 9:6–29 (see vv. 24, 25, 26, in addition to vv. 7 and 11). Second, Paul has reintroduced one of the critical phrases in the letter, out of works. It is an important phrase in the developing argument (e.g., 9:32, 11:6). Out of works is contrasted here with out of calling, not with out of faith as earlier in the letter. The issue is the calling of God, not making righteous by faith. Paul rejects both salvation by birth (Isaac), and salvation by works (Jacob).
Election (i.e., salvation) is based solely on God’s call, and has been so from the beginning of Israel’s story. Third, election language here is corporate language, not individual. Paul is talking in salvation history terms: God chose a people through Isaac rather than Ishmael and through Jacob rather than Esau to be the people of the covenant and of promise.
Paul’s point is clear. Salvation is based exclusively on the call of God. God never promised to save all ethnic Israel, so the rejection of Messiah Jesus by the majority of Jews does not undermine the integrity of God’s word. Paul’s argument suggests that this rejection is a sign that Israel stands outside the Abrahamic covenant and thus the people of God. Israel now stands where Ishmael and Esau once stood.
--Toews, J. E. (2004). Romans. Believers church Bible commentary, Romans 9:6b-13