Galatians 4:23-27

But the son by the slave woman was born according to the flesh, and the son by the free woman through the promise.

This is speaking allegorically, for these women are two covenants: one coming from Mount Sinai giving birth to children who are to be slaves; she is Hagar.

Now this Hagar is Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is enslaved with her children.

But the Jerusalem above is free; she is our mother.

For it is written: “Rejoice, infertile one, you who do not give birth; Break forth and shout, you who are not in labor; For the children of the desolate one are more numerous Than those of the one who has a husband.”