Interestingly, Paul does not take his information about what the law says from the time of Moses, but from the time of Abraham, who lived hundreds of years before the law was given. The facts of history recorded in Genesis were well known to the Galatian churches, and in particular the history of Abraham who was a figure of such significance in Israel.
Hasn’t Paul been emphasising in this epistle that the covenant made with Abraham was a covenant of promise? How can he now also draw lessons about the law and about a legal relationship with God from that same covenant? He has already shown us that the law and the promise are utterly incompatible with each other and cannot be combined together because they provide justification on entirely different terms. And yet here he tells us that Abraham’s two sons represent covenants as different as law and grace. Furthermore, how does the Bible teach us about the law before the great law-giver Moses was even born, and before the covenant of the law was given through his mediatorial work? The answer to this question will shed light not only on the law, but on the nature of the Abrahamic covenant with its two distinct parts, for by the wisdom of God the physical children of Abraham included both a child of promise and a child of the flesh. The one, Isaac, represents those who are of faith and born of the Spirit in all ages, and the other, Ishmael, represents those who are under the law and born of the flesh in all ages. So then, even before the law was given through Moses, God gave this profound picture of these two types of man, for the man of flesh did not first make his appearance at Sinai; the natural man who comes into the world with the law written on his heart existed from the time of the fall onwards.