What does Galatians 4:25 mean?

Paul is creating a specific and careful allegory about two covenants. His purpose is to show the difference between attempting to obtain salvation by works, as opposed to accepting it on the basis of faith alone. Paul uses Abraham's two sons, Ishmael (Genesis 16:1–5) and Isaac (Genesis 21:1–3), to make his point. In the previous verse, he wrote that Abraham's slave-wife Hagar represents the covenant God made with Israel at Mount Sinai. This is the covenant of the law of Moses. Just as Hagar's child with Abraham was born into slavery, since she was a slave, all born under the covenant of the law are also born into slavery to their own sinfulness.

Now Paul adds another layer onto the allegory. Hagar, symbolic of the Mount Sinai covenant (Exodus 19), also corresponds to the Jerusalem of Paul's day, the seat of Jewish religious authority. The false teachers misleading the Galatian Christians were from Jerusalem. As representatives of the law, they were leading the Galatians into a system of slavery to their sin. Adding to the power of Paul's allegory, the Jerusalem of his day was also slave to the Roman government. It was a place under both political and spiritual slavery.

In the following verse, Paul will point to another Jerusalem, one that is free.