Paul makes the argument that the problem that Israel as a whole has not believed is not because God's word or promise has failed, but that just because someone is descended from Abraham does not automatically make them a "true Israelite." Not every descendant gets an automatic pass. His illustration is Abraham had two sons, Isaac and Ishmael, but only one was the child of promise. Isaac had two sons, Esau and Jacob, but only one was the child of promise.