Robert Blair's The Grave
Heaven's Portals Wide Expand to Let Him In
Blake uses several strategies in The Marriage of Heaven and Hell to acquaint his reader with the level of perception which he has called 'fourfold vision.' In this passage he first associates this expanded perception with gaining access to the 'tree of life' which man lost when he was expelled from the Garden of Edan. He tells us that the appearance of creation will be changed from its limited, damaged perception. He provides two metaphors for the process of apprehending the infinite, holy world: through removing the surfaces which overlay and obscure the real, and by cleaning the channels which have been provided to give access.
Marriage of Heaven & Hell, Plate 14, (E 39) "The ancient tradition that the world will be consumed in fire at the end of six thousand years is true. as I have heard from Hell. For the cherub with his flaming sword is hereby commanded to leave his guard at the tree of life, and when he does, the whole creation will be consumed, and appear infinite. and holy whereas it now appears finite & corrupt. This will come to pass by an improvement of sensual enjoyment. But first the notion that man has a body distinct from his soul, is to be expunged; this I shall do, by printing in the infernal method, by corrosives, which in Hell are salutary and medicinal, melting apparent surfaces away, and displaying the infinite which was hid. If the doors of perception were cleansed every thing would appear to man as it is: infinite. For man has closed himself up, till he sees all things thro' narrow chinks of his cavern."Blake's emphasis in the following passage is gaining the ability to see the infinite always, not just some times in some things. The 'firm perswasion' is said to capable of moving mountains as is faith in these Biblical passages.
 Then came the disciples to Jesus apart, and said, Why could not we cast him out?
 And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.
 For verily I say unto you, That whosoever shall say unto this mountain, Be thou removed, and be thou cast into the sea; and shall not doubt in his heart, but shall believe that those things which he saith shall come to pass; he shall have whatsoever he saith.
First Corinthians 13
 And though I have the gift of prophecy, and understand all mysteries, and all knowledge; and though I have all faith, so that I could remove mountains, and have not charity, I am nothing.
Marriage of Heaven & Hell, Plate 12, (E 38)
"Isaiah answer'd. I saw no God. nor heard any, in a finite
organical perception; but my senses discover'd the infinite in
every thing, and as I was then perswaded. & remain confirm'd;
that the voice of honest indignation is the voice of God, I cared
not for consequences but wrote.
Then I asked: does a firm perswasion that a thing is so, make it so?
He replied. All poets believe that it does, & in ages of imagination
this firm perswasion removed mountains; but many are not capable
of a firm perswasion of any thing. ...
I then asked Ezekiel. why he eat dung, & lay so long on his
right & left side? he answerd. the desire of raising other men
into a perception of the infinite this the North American tribes
practise. & is he honest who resists his genius or conscience.
only for the sake of present ease or gratification?"
Blake succinctly expresses the consequences of perceiving without the blinders of the conventional thought presented by the Angel.
Marriage of Heaven & Hell, Plate 25, (E 45)
"For every thing that lives is Holy"
Here are additional quotes from The Marriage of Heaven and Hell which expand the idea of spiritual perception.
"How do you know but ev'ry Bird that cuts the airy way, Is an immense world of delight, clos'd by your senses five?"
"Eternity is in love with the productions of time." "The hours of folly are measur'd by the clock, but of wisdom: no clock can measure." "The most sublime act is to set another before you." "Every thing possible to be believ'd is an image of truth." "The soul of sweet delight. can never be defil'd," "When thou seest an Eagle, thou seest a portion of Genius. lift up thy head!" "Improvement makes strait roads, but the crooked roads without Improvement, are roads of Genius." "Thus men forgot that All deities reside in the human breast." "The worship of God is. Honouring his gifts in other men each according to his genius. and loving the greatest men best, those who envy or calumniate great men hate God, for there is no other God."Martin Nurmi in The Norton Library's Blake's Poetry and Designs shares this insight:
"If everyone learned to see this way...there would be a general transformation of men's view of this world. There would also, as a consequence of this view, be a literal transformation of this world, an apocalypse. Viewing this world as 'One continued Vision or Fancy.' and knowing that life is truly a divine - and human - unity in Christ, men would re-establish society on a new foundation, forming laws of freedom and love rather than repression, abolishing every form of tyranny that prevents man from realizing his potentialities, and celebrating the divinity that is in every man." (Page 557)