For they are not all Israel
The distinction is between Israel after the flesh, the mere natural posterity of Abraham, and Israelites who, through faith, are also Abraham's spiritual children. Gentiles who believe are also of Abraham's spiritual seed; but here the apostle is not considering them, but only the two kinds of Israelites, the natural and the spiritual Israel. (Romans 4:1-3); (Galatians 3:6); (Galatians 3:7); (John 8:37-39).
( See Scofield) - (Romans 11:1).