{Which is our mother} (h(8874)is estin m(8874)(8872) h(886d)(936e)). 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  Isa 54:1 , a song of
triumph looking for deliverance from a foreign yoke. {Rejoice}
(euphranth(8874)i). First aorist passive imperative of euphrain(935c).
{Break forth} (
(8878)on). First aorist active imperative of

(8867)numi, to rend, to burst asunder. Supply euphrosun(886e) (joy)
as in  Isa 49:13 . {The desolate} (	(8873) er(886d)ou). 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.