Albert Barnes Commentary Romans 9:7

"neither, because they are Abraham`s seed, are they all children: but, In Isaac shall thy seed be called." — Romans 9:7 (ASV)

Are they all children? Adopted into the true family of God. Many of the descendants of Abraham were rejected.

But in Isaac. This was the promise (Genesis 21:12).

Shall thy seed... This means Your true people. This implied a selection, or choice; and therefore, the doctrine of election was illustrated in the very commencement of the history of the nation. As God had then made such a distinction, He might still do it. As He had then rejected a part of the natural descendants of Abraham, so He might still do it. This is the argument which the apostle is pursuing.