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.