Which is our mother (hti estin mhthr hmwn). The mother of us Christians, apply the allegory of Hagar and Sarah to us. The Jerusalem above is the picture of the Kingdom of God. Paul illustrates the allegory by quoting Isaiah 54:1 , a song of triumph looking for deliverance from a foreign yoke. Rejoice (eupranqhti). First aorist passive imperative of euprainw. Break forth (rhxon). First aorist active imperative of rhgnumi, to rend, to burst asunder. Supply euprosunhn (joy) as in Isaiah 49:13 . The desolate (th erhmou). The prophet refers to Sarah's prolonged barrenness and Paul uses this fact as a figure for the progress and glory of Christianity (the new Jerusalem of freedom) in contrast with the old Jerusalem of bondage (the current Judaism). His thought has moved rapidly, but he does not lose his line.