Blake's familiar hymn from plate 1 of Milton uses the symbol of Jerusalem to represent the ideal spiritual condition, and England to represent the fallen, material condition.
Milton, Plate 1, (E 95)
"And did those feet in ancient time,
Walk upon Englands mountains green:
And was the holy Lamb of God,
On Englands pleasant pastures seen!
And did the Countenance Divine,
Shine forth upon our clouded hills?
And was Jerusalem builded here,
Among these dark Satanic Mills?
Bring me my Bow of burning gold:
Bring me my Arrows of desire:
Bring me my Spear: O clouds unfold!
Bring me my Chariot of fire!
I will not cease from Mental Fight,
Nor shall my Sword sleep in my hand:
Till we have built Jerusalem,
In Englands green & pleasant Land."
In Galatians Paul uses the symbols of Abraham's two sons to represent the same dichotomy of spirit and flesh: one by a freewoman and one by a bondwoman. Paul presents this as allegory of the two covenants; the one of bondage the other of freedom. He calls the condition of freedom Jerusalem but specifies that he speaks of the Jerusalem which is above: the mother of all.
The same desire that Blake has for building Jerusalem in England is expressed as Paul's desire that Christ be formed in those he addresses as little children. Paul calls the children of the freewoman the children of promise.
[ 19] My little children, of whom I travail in birth again until Christ be formed in you,
 I desire to be present with you now, and to change my voice; for I stand in doubt of you.
 Tell me, ye that desire to be under the law, do ye not hear the law?
 For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.
 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.
 Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.
 For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.
 But Jerusalem which is above is free, which is the mother of us all.
 For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath an husband.
 Now we, brethren, as Isaac was, are the children of promise.
 But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.
 Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman.
 So then, brethren, we are not children of the bondwoman, but of the free.
Blake on plate 77 of Jerusalem seems to have written a continuation of the poem in Milton. In it Blake has Jerusalem call to her sister England to awake. He again recalls the ancient time of joy and love which can return if the Lamb of God but be received.
Jerusalem, PLATE 77, (E 231)
"England! awake! awake! awake!
Jerusalem thy Sister calls!
Why wilt thou sleep the sleep of death?
And close her from thy ancient walls.
Thy hills & valleys felt her feet,
Gently upon their bosoms move:
Thy gates beheld sweet Zions ways;
Then was a time of joy and love.
And now the time returns again:
Our souls exult & Londons towers,
Recieve the Lamb of God to dwell
In Englands green & pleasant bowers. "
Image from Blake's illustrations for Milton's Ode on the Morning of Christ's Nativity
Painted for Thomas Butts in 1815