Paul speaks directly to those who wanted to put themselves under the law. If they want the law so much, have they really listened to it? The law itself, in its own story of Abraham's family, warns against the very bondage they are choosing.

Barnes explains that "law" here means the books of the law, not only the commands given at Sinai; the sense is, "Will you not listen to a narrative found in one of the books of the Law itself." Gill likewise points them to "the five books of Moses," and says that wanting to be under the law as a covenant of works "was downright madness."

Paul answers by telling them what the law says: "Abraham had two sons, the one by a bondmaid, the other by a freewoman" (Galatians 4:22). He explains, "these are the two covenants; the one from the mount Sinai, which gendereth to bondage" (Galatians 4:24), while "Jerusalem which is above is free, which is the mother of us all" (Galatians 4:26). He ends with the Scripture's own verdict, "Cast out the bondwoman and her son" (Galatians 4:30), and the conclusion, "we are not children of the bondwoman, but of the free" (Galatians 4:31).

Among the Jews, "the law" could mean the five books of Moses as a whole, and Barnes says the word refers here to "the book of the Law" (the passage itself is in Genesis), "not to the commands that were uttered on Mount Sinai." The story Paul tells is in Genesis: Hagar, Sarai's Egyptian handmaid, bore Ishmael to Abram (Genesis 16:1, 15), and Sarah later bore Isaac "at the set time of which God had spoken" (Genesis 21:2). Sarah's words, "Cast out this bondwoman and her son" (Genesis 21:10), are quoted by Paul in Galatians 4:30.

Paul turns the very law the Galatians wanted against their mistake. The law they wanted to live under tells of two sons: one born "after the flesh," the other "by promise" (Galatians 4:23). Only the son of promise was the heir. Matthew Henry calls Hagar and Sarah apt emblems of the two different dispensations of the covenant, one of bondage and one of freedom.

This shows how God works. The promised son came by God's word and power, not by human effort, just as believers become God's children by promise. "Now we, brethren, as Isaac was, are the children of promise" (Galatians 4:28). Listening carefully to the law leads, in the end, to grace.