Chapter four continues section two and the explanation of the inheritance available to all who believe in Jesus. Galatians 4:1-7 contain the wonderful truth that Jesus was born at just the right time in just the right manner to accomplish everything God wanted to do, namely, to adopt rebel humans back into his spiritual family and make them free. This is important because we are all enslaved in sin by nature (Galatians 4:8-12), but in Christ we are freed from that. Paul wondered, then, why someone would place himself under any kind of restrictions again.
Galatians 4:13-20 break from Paul’s explanation of his doctrine to a personal appeal to his initial readers. He reminded them of how they had received him. Even though he was violently ill, they were not repulsed by him, but rather received him and his message as if he were Jesus himself. In fact, Paul noted that they would have gouged out their own eyes and given them to him if they were able. (This offhand remark in Galatians 4:15 possibly hints to Paul’s “thorn in the flesh,” a constant reminder of his weakness (2 Corinthians 12:7-10).)
The chapter (and second section) closes with an allegory (an extended metaphor), in which Paul likened the old covenant to Hagar and Ishmael. While he was a legitimate son of Abraham, Ishmael was not the son through whom the promises would be given. In the same way, while the Law, symbolized by Mt. Sinai, was legitimately God’s way of leading Israel, it was never meant to bring righteousness or salvation. As Hagar and Ishmael were slaves in Abraham’s household, those who subject themselves to the Law are slaves to it.
Sarah and Isaac (along with Mt. Zion), on the other hand, represent the new way through Jesus. As they were freepersons in Abraham’s household, those who come to God through faith in Jesus find freedom from the Law. Why, then, would someone place himself back into slavery when he had been set free?