What does Galatians 4:22 mean?

In this and the following verses, Paul will continue to make the case that the only way to be made right with God—to be "justified"—is by faith in Christ and not by following the works of the law.

Paul turns again to the figure of Abraham, building a somewhat complicated illustration to make his point. Since the Galatians are putting so much stock in the law-based teaching of the Judaizers, Paul will also build a case from the Old Testament Scriptures. He begins by saying "it is written," invoking the divinely inspired Scriptures as his evidence.

Paul mentions Abraham's two sons, one by a slave and one by a free woman. For the purposes of Paul's illustration, he is not including the sons born to Abraham much later. The free woman is Sarah, formerly Sarai, Abraham's wife. The slave woman is Hagar, Sarah's own Egyptian slave-girl.

Abraham and Sarah had waited many, many years for God's promise of a child to be fulfilled. Finally, Sarah gave in to impatience and gave her servant Hagar to Abraham as a slave-wife, so she might have a child by proxy (Genesis 16:2). Hagar's son, born in this way, was Ishmael (Genesis 16:5). Eventually, though, Sarah did indeed have her own birth son, Isaac (Genesis 21:1–3).

Paul will use the differences between Ishmael and Isaac to illustrate the distinction between trying to earn salvation by works, as opposed to accepting salvation by faith.