Tell me now, you who have become so enamored with the law: Have you paid close attention to that law? Abraham, remember, had two sons: one by the slave woman and one by the free woman. The son of the slave woman was born by human plotting; the son of the free woman was born by God’s promise. This illustrates the very thing we are dealing with now. The two births represent two ways of being in relationship with God. One is from Mount Sinai in Arabia. It corresponds with what is now going on in Jerusalem—a slave life, producing slaves as offspring. This is the way of Hagar. In contrast to that, there is an invisible Jerusalem, a free Jerusalem, and she is our mother—this is the way of Sarah. Remember what Isaiah wrote:
Rejoice, barren woman who bears no children,
shout and cry out, woman who has no birth pangs,
Because the children of the barren woman
now surpass the children of the chosen woman.
Isn’t it clear, friends, that you, like Isaac, are children of promise? In the days of Hagar and Sarah, the child who came from faithless plotting (Ishmael) harassed the child who came—empowered by the Spirit—from the faithful promise (Isaac). Isn’t it clear that the harassment you are now experiencing from the Jerusalem heretics follows that old pattern? There is a Scripture that tells us what to do: “Expel the slave mother with her son, for the slave son will not inherit with the free son.” Isn’t that conclusive? We are not children of the slave woman, but of the free woman.