A VISION

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Wikimedia Commons
The Last Judgment
Commissioned by Countess of Egremont
1808

Blake's image of The Last Judgment can be seen as the cycle of creation, fall and redemption seen from the perspective of Eternity. In describing his image in prose Blake makes this statement: (I've added punctuation for ease of reading.)

A Vision of the Last Judgment, (E 563)
"Creation which was an act of Mercy. I have represented
those who are in Eternity by some in a Cloud within the Rainbow
that Surrounds the Throne. They merely appear as in a Cloud when
any thing of Creation Redemption or Judgment are the Subjects of
Contemplation. Tho their Whole Contemplation is Concerning these
things, the reason they so appear is The Humiliation of the
Reasoning & Doubting Selfhood & the Giving all up to Inspiration.
    By this it will be seen that I do not consider either the Just   
or the Wicked to be in a Supreme State but to be every one of
them States of the Sleep which the Soul may fall into in its
Deadly Dreams of Good & Evil when it leaves Paradise
following the Serpent" 

From the point of view of those in Creation, those in Eternity are unclear, as in a cloud, but actually they surround the fundamental reality which is represented by the throne on which Jesus is seated. The cloud is within the rainbow because the hope of redemption is an ever present promise to humanity.
Since Blake is presenting the cycle through which man travels in his material existence, the realities of Eternity are masked or obscured to the individual. The loss of the Divine Vision has led to the dependence of man on his Reasoning & Doubting Selfhood

The Everlasting Gospel, (E 520)
"Reasoning upon its own Dark Fiction
In Doubt which is Self Contradiction
Humility is only Doubt
And does the Sun & Moon blot out
Rooting over with thorns & stems     
The buried Soul & all its Gems
This Lifes dim Windows of the Soul
Distorts the Heavens from Pole to Pole
And leads you to Believe a Lie
When you see with not thro the Eye   
That was born in a night to perish in a night
When the Soul slept in the beams of Light."

But the world through which man travels is not as it seems.  When man mistakes the states through which he travels as the essential man, he is falling into the deadly dream of good and evil by following the serpent who leads man away from the ability to perceive the Divine Vision. Although we see pictured in the image, man in various times and conditions, Blake tells us that these are not individuals but symbols for the states experienced in life.  

A Vision of the Last Judgment, (E 556)
"it ought to be understood that the Persons
Moses & Abraham are not here meant but the States Signified by
those Names the Individuals being representatives or Visions of
those States as they were reveald to Mortal Man in the Series of
Divine Revelations. as they are written in the Bible these
various States I have seen in my Imagination when distant they
appear as One Man but as you approach they appear
Multitudes of Nations."

Blake is trying to reach men on whatever level of development they have attained. His symbolic poetry aims at a different audience. His picture of The Last Judgment is saying to those who can read the picture language such as was used by Michelangelo or in the stained glass windows of the Gothic cathedrals, that Creation is a Mercy designed to repair the damage of the Fall: that whatever adversity or advantage man experiences in this life there is provision made for re-entry into Paradise. 
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