What does Galatians 4:26 mean?

The Galatian Christians are being deceived into thinking they need to follow the law of Moses, as well as believe in Christ, in order to be saved (Galatians 2:4). Calling on the story of Abraham, Paul is refuting this idea by creating an allegory of pairs:

Abraham had two wives, Hagar and Sarah. Hagar was a slave, and Sarah was free.

Abraham had two sons, Ishmael and Isaac. Ishmael, born to the slave, was also a slave (Genesis 16:1–3). Isaac, born to the free woman, was also free (Genesis 21:1–3).

God made two covenants. Hagar represents the one given at Mount Sinai with the law of Moses. All born under the law are like children born to a slave woman; they are born into slavery themselves. God's covenant with Abraham, however, promised freedom.

And now, according to Paul's analogy, there are two Jerusalems. Hagar also corresponds to the Jerusalem of Paul's day. It is under the slavery of the Roman government, but it is also the seat of Jewish power, representing the law of Moses. In that way, it is the Jerusalem under slavery.

There is another Jerusalem, though. Paul describes it as being "above" and being free. He writes that Jerusalem is our mother. The implication is that as Sarah, the free woman, is mother to Isaac, this "above Jerusalem," the free one, is mother to those who trust in Christ to save them.

Other New Testament writers describe a new Jerusalem, as well. Hebrews 12:22 calls it the "heavenly Jerusalem" and the "city of the living God." This free Jerusalem is where God Himself lives right now. John describes the day when all will be made right once and for all and this "new Jerusalem" will come down out of heaven. On that day, God will come to live with His people (Revelation 21:2–3).