In this passage, Paul makes a very interesting allegory to drive home a simple point: We are children of God based on His promise, not children of God based on our keeping his law.
He talks about the story of Hagar and Sarah found in Genesis 16-21. God had promised to give Abraham a son, but after years of waiting, Abraham and Sarah had started to lose hope that God would keep his promise. So Sarah suggested that Abraham have a child through her slave Hagar (something atrocious to us, but perfectly normal back in those days).
Through Hagar, Abraham got his first son, Ishmael. But this was a son that came not based on the promise of God and his provision. Rather, it was based solely on human efforts. Later though, Sarah did give birth to a son named Isaac. His birth was a total miracle, a total act of God, as Sarah was 90 years old when she gave birth. And it was through Isaac, God told Abraham, that He would keep his promise to make Abraham into a great nation.
Paul then says those who try to be justified by the law are symbolized by Hagar and her son Ishmael. They are not trying to receive the blessing of God based on God’s promise and God’s work. Rather, they are trying to achieve it through their own human effort.
But there’s a problem with this. Children born of a slave are slaves themselves. So people who try to be “children of Hagar,” justified by their own human efforts, will in reality only find themselves enslaved by the law of sin and death. In other words, the law can’t save them at all. All it does is point out their sin and condemn them to death. (4:24-25)
On the other hand those who are trying to be justified before God by his grace are like Isaac, children and heirs of God based on God’s promise and God’s work. Because of that, we are no longer enslaved by the law of sin and death. We have been set free, and are now true children of the most high God. (4:26-28)
But just as Ishmael, the child born of human efforts, persecuted Isaac, the child born of God’s promise, so the Judaizers persecuted the Christians. In particular, the Judaizers tried to shut out the Galatian Christians until they agreed to put themselves under slavery to the law like the Judaizers were. (4:29)
So Paul speaks very strongly here:
What does the Scripture say? “Get rid of the slave woman and her son, for the slave woman will never share in the inheritance with the free woman’s son.” (4:30)
In short, “Get rid of these false teachers. They are children of the slave. And they will never share in your inheritance. They have no part with you. They are trying to exclude you when the reality is that it is they who are excluded.”
And then he reemphasizes,
“Therefore, brothers, we are not children of the slave woman, but of the free woman.” (4:31)
He then charges them,
“It is for freedom that Christ has set us free. Stand firm, then, and do not let yourselves be burdened again under a yoke of slavery.” (5:1)
Let us never forget that. Christ set us free from the law, not so that we would go under it all over again, but that we would truly be free from it forever. He set us free so that we could live as children of God, knowing that we are already accepted by Him, and not worrying about whether we are good enough.
How about you? Are you living with the peace and joy of a child of God? Or are you still living like a slave burdened by all the rules of religion?