In Chapter 4 of Galatians, Paul continues to demolish the idea that we can earn acceptance from God through human effort. He reminds the Galatians that the way to walk with God is through faith and takes them back to the Old Testament to prove his point. His primary example is the difference between Hagar and Sarah, one of whom bore a son by natural means (human effort) and the other a son of the promise (by a miracle of God).

Slavery and sonship are powerful images the bible uses to describe the ways people relate to God. The mindset of a slave is very different to the mindset of a son. The slave has obligations but no privileges. The son has rights and receives an inheritance.