John Calvin 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)

But, “In Isaac shall your seed be called.” Paul mentions this to show that the hidden election of God overrules the outward calling, and that it is still by no means inconsistent with it; on the contrary, it tends to its confirmation and completion. So that he might then prove both in an orderly way, he first assumes that the election of God is not tied to the natural descendants of Abraham, and that it is not something included in the conditions of the covenant; and he now confirms this with a most suitable example.

For if there should have been any natural progeny who did not fall away from the covenant, this should have been especially the case for those who first obtained the privilege. But when we find that of Abraham’s first sons—while he was still alive and the promise was new—one of them was separated as the seed, how much more might the same thing have taken place among his distant descendants?

Now this testimony is taken from Genesis 17:20, where the Lord gives an answer to Abraham, stating that He had heard his prayer for Ishmael, but that there would be another on whom the promised blessing would rest. It therefore follows that some people are by special privilege elected out of the chosen people, and in them the common adoption becomes efficacious and valid.