What does Galatians 4:27 mean?
To explain why salvation by faith alone is God's actual plan for mankind, rather than salvation by works and rituals, Paul is creating an allegory. He compares the slavery of those who follow the law with the freedom of those who come to God by faith in Christ. He has compared those born to a slave women, such as Abraham's wife Hagar (Genesis 16:1–5), to those who live under the law.Those born to the free woman Sarah, however, live in freedom. He will go on to show that this freedom comes from being born according to God's Spirit: through faith in Christ.
Here however, Paul quotes from Isaiah 54:1 and applies it to his allegory. Isaiah was prophesying about Israel's future after her captivity to Babylon. She would once again thrive and grow, as a formerly barren woman who now begins to have many children.
Paul connects this prophesy to his allegory about Abraham's wife Sarah. She waited and waited and finally gave birth to a free son of her own, just as God had promised (Genesis 21:1–3). In a similar way, when the time was just right, God sent Christ to earth as a man to provide a way for those under the law to be truly free (Galatians 3:23–29).