Romans 9:7–8:

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.” In other words, it is not the natural children who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring.(NIV)

nor are they all children because they are Abraham’s descendants, but: “THROUGH ISAAC YOUR DESCENDANTS WILL BE NAMED.” That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants. (NASB)

Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed. (KJV)

In Rom 9:6, Paul argues that “God’s word does not fail” when a Jew can be found that will not inherit the the promised glory of God as co-heirs with Christ by saying that “Not all who are descended from Israel are Israel” (similar to the arguments of Rom 2:28–29).

Paul now further builds this argument (via the testimony of Scripture): “‘Through Isaac [Abraham's] descendants will be named’ (Gen. 21:12); therefore, not all those who are ‘descendants of Abraham’ are also ‘Abraham’s children’.”

To be absolutely clear, Paul states the obvious sub-conclusion to this sub-argument: “‘Not all who are descended from Israel are Israel’ (Rom 9:6); likewise, all those who are ‘descendants of Abraham’ are also ‘Abraham’s children’ (Rom 9:7); therefore, ‘it is not the natural children who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring,’ i.e., ‘God’s children’ (Rom 9:8).”