Paul uses the Judaizers own claim against them. They claimed that by virtue of their birth, begin Abraham's descendants that they are God's people. Paul uses this argument against them by telling them Abraham had two sons one by flesh and one by supernatural birth. His conclusion is that the Judaizers were born after the flesh and like Ishmael had no part of the promise, but those who had believed in the Lord Jesus Christ are like Isaac, children of the promise. This is a very clear distinction between being religious and righteous, living by law and living by the faith. Which are living according too?