The reading is from St. Paul's Letter to the Galatians 4:22-27

Brethren, Abraham had two sons, one by a slave and one by a free woman. But the son of the slave was born according to the flesh, the son of the free woman through promise. Now this is an allegory: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar. Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children. But the Jerusalem above is free, and she is our mother. For it is written, "Rejoice, O barren one who does not bear; break forth and shout, you who are not in travail; for the children of the desolate one are many more than the children of her that is married."