"Alright, you want to be under the law? Then actually listen to what the law says.
Abraham had two sons. One was born to a slave woman — Hagar. The other was born to a free woman — Sarah. The slave woman's son was born through ordinary human effort. The free woman's son was born through God's promise.
Here's the deeper meaning: these two women represent two Covenants. Hagar represents the covenant from Mount Sinai — and her children are born into slavery. Hagar corresponds to the present Jerusalem, because Jerusalem and her children are currently in bondage.
But the Jerusalem above is free — and she is our mother.
As the scripture says: 'Rejoice, you who never bore children! Break out in celebration, you who never went through labor! Because the desolate woman will have more children than the woman who has a husband.'"