Romans 9:7

Neither because they are the seed of Abraham, etc. - Nor can they conclude, because they are the natural descendants of Abraham, that therefore they are all of them, without exception, the children in whom the promise is to be fulfilled.

But, in Isaac shall thy seed be called - The promise is not confined to immediate natural descent, but may be accomplished in any part of Abraham's posterity. For Abraham had several sons besides Isaac, Genesis 25:1, Genesis 25:2, particularly Ishmael, who was circumcised before Isaac was born, and in whom Abraham was desirous that the promise should be fulfilled, Genesis 17:18, and in him God might have fulfilled the promise, had he so pleased; and yet he said to Abraham, Genesis 21:12 : Not in Ishmael, but in Isaac, shall thy seed be called.

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 ... - Thy 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; and 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.

9:7 Neither because they are lineally the seed of Abraham, will it follow that they are all children of God - This did not hold even in Abraham's own family; and much less in his remote descendants. But God then said, In Isaac shall thy seed be called - That is, Isaac, not Ishmael, shall be called thy seed; that seed to which the promise is made.