In Galatians 4:21-31, Paul gave an interpretation of the story from the Old Testament about Abraham and the two sons he had, one to Hagar the slave woman, and the other to Sarah his wife.  The false teachers in Galatia were perhaps using this story to try to force non-Jewish believers to become Jews.  However, Paul showed how the story itself supports his main thesis that both Jews and non-Jews may receive God’s promised blessing through faith, not through law-keeping.