This book is about children of promise verses children of the flesh; natural Israel verses spiritual Israel; the seed of the Serpent verses the seed of Christ and of the woman. God has never had two separate and distinct peoples, one earthly and of an earthly kingdom, and one heavenly and of a heavenly kingdom. Within natural Israel there has always been a spiritual remnant chosen by God's grace alone, through faith alone, in Christ alone. Natural Israel is not God's glory, or those upon whom God's Glory rests upon, but His Glory rests upon His spiritual seed or "children of promise" who are born according to the Spirit of grace (Rom. 9:8). And even of the Gentiles, Paul says, like Isaac, we too are called "children of promise" (Gal. 4:28). Natural Israel only according to the flesh, says Paul, like Ishmael, is to be "cast out," never to share in the inheritance with the free woman (Sarah) and her son (Isaac).