Although the children of Israel could have been free if they had stuck with God’s covenant, they rejected him, so they became slaves of the nations, first the Babylonians and then the Romans. This explains the meaning of the parable of Hagar and Sarah. The child of Hagar is a slave, and the child of the other is the heir of promises (Gal 4:21-23).
These things are being taken figuratively: The women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar. Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children. But the Jerusalem that is above is free, and she is our mother (Gal 4:24-26).Most commentators assume that Hagar is the old covenant and Sarah represents the new, but that is not what Paul is saying. Hagar represents the current situation of Israel. They have chosen to be live under the Mosaic Covenant, but have refused to follow God, so they were enslaved by the Romans. Their situation would be made worse when the Romans destroyed the temple, because that put in an end to the temple sacrifices.
As the chosen people, they should have been first to receive the promise, but they chose to reject Jesus, although he had fullfiled the covenant on their behalf.
Christ redeemed us from the curse of the law by becoming a curse for us... so that by faith we might receive the promise of the Spirit (Gal 3:13-14).Jesus had taken the curse on behalf of the people of the covenant, but they refused to accept it, so they remained stuck under its curse of the law. If they had accepted him, they could have received the full blessing promised through Moses, plus the additional blessing of the gift of the Spirit. By rejecting what God was doing though, they had put themselves under the curse of the law. The consequence was slavery under the Romans.
Paul reminded the Christians in Galatia that they were children of promise.
Therefore, brothers and sisters, we are not children of the slave woman but of the free woman (Gal 4:31).As children of promise, who have been set free from the power of sin, they should avoid placing themselves under the authority of those who are not free. Jesus has set them free from the powers of evil, so they should have nothing to do with people who would put them back into slavery. This is not just a matter of foolish rules. By submitting to the requirements of the Jews, they would be putting themselves back under the authority of the powers of evil.