The phrase Lamb of God appears only twice in the Bible, both times in the Gospel of John.
 The next day John [the Baptist] seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.
 And looking upon Jesus as he walked, he saith, Behold the Lamb of God!
Most of the occurrences of the word lamb in the New Testament are in the Book of Revelations. The lamb of the Book of Revelations is unique to the vision of John of Patmos, incorporating aspects of the Christ but including characteristics of the of a vindictive God. The use of Lamb of God by Blake is distinctive also. Blake uses the term Lamb of God to express the eternal dimension of Jesus who is the incarnation. The Lamb of God is the essence being incarnated.
John in his gospel calls attention to Lamb of God as he who takes away the sins of the world. Blake's desire to emphasize the concept of forgiveness rather than repentance or retribution make the Lamb of God the perfect symbol for the essence of God's activity among men. God's Lamb accepted the role of assuming the robes of blood sacrificing his position among the Eternals to live in generation among men.
For Blake the Lamb of God is not the manifest incarnation; that is Jesus whose life in the world of generation is recorded in the Gospels.
Percival in William Blake's Circle of Destiny explains the relationship between Urthona and Los as similar to that between the Lamb of God and Jesus. "Los is finite and temporal; Urthona is eternal. Urthona is the capacity and indomitable will to imagine which makes man human. In the exercise of that capacity man falls to the level of Los, but the capacity remains unimpaired...[Urthona] provides an imperishable reality independent of all that is finite and temporal." (Page 144)
Milton, PLATE 13 , (E 107)
"For then the Body of Death was perfected in hypocritic holiness,
Around the Lamb, a Female Tabernacle woven in Cathedrons Looms
He died as a Reprobate. he was Punish'd as a Transgressor!
Glory! Glory! Glory! to the Holy Lamb of God
I touch the heavens as an instrument to glorify the Lord!"
Milton, PLATE 14 , (E108)
"The Lamb of God is seen thro' mists & shadows, hov'ring
Over the sepulchers in clouds of Jehovah & winds of Elohim
A disk of blood, distant; & heav'ns & earth's roll dark between"
Jerusalem, Plate 7, (E 150)
"And the Religion of Generation which was meant for the destruction
Of Jerusalem, become her covering, till the time of the End.
O holy Generation! [Image] of regeneration!
O point of mutual forgiveness between Enemies!
Birthplace of the Lamb of God incomprehensible!
The Dead despise & scorn thee, & cast thee out as accursed:
Seeing the Lamb of God in thy gardens & thy palaces:
Where they desire to place the Abomination of Desolation."
Jerusalem, Plate 20, (E 165)
"Albion lov'd thee! he rent thy Veil! he embrac'd thee! he lov'd thee!
Astonish'd at his beauty & perfection, thou forgavest his furious love:
I redounded from Albions bosom in my virgin loveliness.
The Lamb of God reciev'd me in his arms he smil'd upon us:
He made me his Bride & Wife: he gave thee to Albion.
Then was a time of love: O why is it passed away!"
Jerusalem, Plate 25, (E 170)
"As the Sons of Albion have done to Luvah: so they have in him
Done to the Divine Lord & Saviour, who suffers with those that suffer:
For not one sparrow can suffer, & the whole Universe not suffer also,
In all its Regions, & its Father & Saviour not pity and weep.
But Vengeance is the destroyer of Grace & Repentance in the bosom
Of the Injurer: in which the Divine Lamb is cruelly slain:
Descend O Lamb of God & take away the imputation of Sin
By the Creation of States & the deliverance of Individuals Evermore Amen"
Four Zoas, PAGE 18, (E 321)
"Why is the Sheep given to the knife? the Lamb plays in the Sun
He starts! he hears the foot of Man! he says, Take thou my wool
But spare my life, but he knows not that winter cometh fast.
They weighd & orderd all & Urizen comforted saw
The wondrous work flow forth like visible out of the invisible
For the Divine Lamb Even Jesus who is the Divine Vision
Permitted all lest Man should fall into Eternal Death
For when Luvah sunk down himself put on the robes of blood
Lest the state calld Luvah should cease. & the Divine Vision
Walked in robes of blood till he who slept should awake"
Four Zoas, PAGE 104 (FIRST PORTION), (E 376)
"And Enitharmon namd the Female Jerusa[le]m the holy
Wondring she saw the Lamb of God within Jerusalems Veil
The divine Vision seen within the inmost deep recess
Of fair Jerusalems bosom in a gently beaming fire
Then sang the Sons of Eden round the Lamb of God & said
Glory Glory Glory to the holy Lamb of God
Who now beginneth to put off the dark Satanic body
Now we behold redemption Now we know that life Eternal
Depends alone upon the Universal hand & not in us
Is aught but death In individual weakness sorrow & pain
The Lamb of God descended thro the twelve portions of Luvah
Bearing his sorrows & rec[iev]ing all his cruel wounds"
Four Zoas, PAGE 110  (FIRST PORTION), (E 379)
"Thus was the Lamb of God condemnd to Death
They naild him upon the tree of Mystery weeping over him
And then mocking & then worshipping calling him Lord & King
Sometimes as twelve daughters lovely & sometimes as five
They stood in beaming beauty & sometimes as one even Rahab
Who is Mystery Babylon the Great the Mother of Harlots"
Four Zoas, Page 110 , (E 379)
"Jerusalem saw the Body dead upon the Cross She fled away
Saying Is this Eternal Death Where shall I hide from Death
Pity me Los pity me Urizen & let us build
A Sepulcher & worship Death in fear while yet we live
Death! God of All from whom we rise to whom we all return
And Let all Nations of the Earth worship at the Sepulcher
With Gifts & Spices with lamps rich embossd jewels & gold
Los took the Body from the Cross Jerusalem weeping over
They bore it to the Sepulcher which Los had hewn in the rock
Of Eternity for himself he hewd it despairing of Life Eternal
But when Rahab had cut off the Mantle of Luvah from
The Lamb of God it rolld apart, revealing to all in heaven
And all on Earth the Temple & the Synagogue of Satan & Mystery"
Four Zoas, PAGE 122, (E 391)
"And the Eternal Man Said Hear my words O Prince of Light
Behold Jerusalem in whose bosom the Lamb of God
Is seen tho slain before her Gates he self renewd remains
Eternal & I thro him awake to life from deaths dark vale
The times revolve the time is coming when all these delights
Shall be renewd & all these Elements that now consume
Four Zoas, Page 122, (E 391)
"Regenerate She & all the lovely Sex
From her shall learn obedience & prepare for a wintry grave
That spring may see them rise in tenfold joy & sweet delight
Thus shall the male & female live the life of Eternity
Because the Lamb of God Creates himself a bride & wife
That we his Children evermore may live in Jerusalem"