City of God

After these things a son was born to Abraham, according to God's promise, of Sarah, and was called Isaac, which means laughter. For his father had laughed when he was promised to him, in wondering delight, and his mother, when he was again promised by those three men, had laughed, doubting for joy; yet she was blamed by the angel because that laughter, although it was for joy, yet was not full of faith. Afterwards she was confirmed in faith by the same angel. From this, then, the boy got his name. For when Isaac was born and called by that name, Sarah showed that her laughter was not that of scornful reproach, but that of joyful praise; for she said, “God has made me to laugh, so that every one who hears will laugh with me.” Then in a little while the bond maid was cast out of the house with her son; and, according to the apostle, these two women signify the old and new covenants—Sarah representing that of the Jerusalem which is above, that is, the city of God.