Tell me—those of you who want to be under the Law—don't you listen to the Law?
It's written that Abraham had two sons, one by the slave woman and one by the free woman.
The son by the slave woman was conceived the normal way, but the son by the free woman was conceived through a promise.
These things are an allegory: the women are two covenants. One is from Mount Sinai, which gives birth to slave children; this is Hagar.
Hagar is Mount Sinai in Arabia, and she corresponds to the present-day Jerusalem, because the city is in slavery with her children.
But the Jerusalem that is above is free, and she is our mother.
It's written: Rejoice, barren woman, you who have not given birth. Break out with a shout, you who have not suffered labor pains; because the woman who has been deserted will have many more children than the woman who has a husband.a28
Brothers and sisters, you are children of the promise like Isaac.
But just as it was then, so it is now also: the one who was conceived the normal way harassed the one who was conceived by the Spirit.
But what does the scripture say? Throw out the slave woman and her son, because the slave woman's son won't share the inheritance with the free woman's son.b31
Therefore, brothers and sisters, we aren't the slave woman's children, but we are the free woman's children.