MHH 19

Continuing from Plate 18:

[PL 19] 
  My friend the Angel climb'd up from his station into the mill;
I remain'd alone, & then this appearance was no more, but I found
myself sitting on a pleasant bank beside a river by moon light
hearing a harper who sung to the harp. & his theme was, The man
who never alters his opinion is like standing water, & breeds
reptiles of the mind.
  But I arose, and sought for the mill, & there I found my
Angel, who surprised asked me, how I escaped?
  I answerd.  All that we saw was owing to your metaphysics: for
when you ran away, I found myself on a bank by moonlight hearing
a harper, But now we have seen my eternal lot, shall I shew you
yours? he laughd at my proposal: but I by force suddenly caught
him in my arms, & flew westerly thro' the night, till we were
elevated above the earths shadow: then I flung myself with him
directly into the body of the sun, here I clothed myself in
white, & taking in my hand Swedenborgs volumes sunk from the
glorious clime, and passed all the planets till we came to
saturn, here I staid to rest & then leap'd into the void, between
saturn & the fixed stars.
  Here said I! is your lot, in this space, if space it may be
calld, Soon we saw the stable and the church, & I took him to the
altar and open'd the Bible, and lo! it was a deep pit, into which
I descended driving the Angel before me, soon we saw seven houses
of brick, one we enterd; in it were a

Erdman 41-2)

By the 'angel' Blake might be referring to the religious establishment.  This whole book is aimed at a severe critique of the Christian religion as he has seen it.

The angel, you recall had driven the devilish Blake to a sorry position (See MHH 18); he climbed up to where he had been.  At his departure Blake found himself in a pleasant position beside a river; a post could be written describing Blake's pleasure and river.
(For the River of Life see Golden String.)

The river strongly contrasts to the storm that the angel had invoked.

We might say the harper was singing 'good tidings of great joy' which we may find in Luke 2:10,
(Blake in fact found them some years later.)

The harp is a sacred instrument going back to the ancient days in Egypt

(Genesis 4:21  And his brother's name was Jubal: he was the father of all such as handle the harp , the instrument David used to accompany his Psalms.)

Jubal and the Harp
widipedia commons

The angel, you recall had been showing the devil 'his eternal lot', and now the devil 'had his inning'.
(In this drama the 'angel' is a symbol of established religion and the devil is the writer of MHH.)

He grabbed the angel and flew with him through the western gate (in this case eternity).  They went out of the world so to speak and then flew right into the 'body of the sun'

The angel in MHH 18 had come to a black sun, while the devils sun is full of Light.
Blake took the heavy weight of Swedenborg's works which 'sunk him from that glorious clime'; he appears to mean that he had studied Swedenborg who espoused the dead theology that Blake had been able to 'kick'.

With this brilliant poetry Blake is autobiographical and theological: the worship of the 'word for word'  Bible on the pulpit becomes a black hole.

This Plate ends with the seven houses, evocative of John's evaluation of the seven churches, to be particularized no doubt in MHH 19.