Three things stand out prominently in this wonderful picture:
On the right is the cave of the nymphs who conduct innocent souls by the northern gate down into mortal life. Below the cave spread across the bottom is the Sea of Time and Space. On the upper left you see a representation of the Heavenly Realm.
Homer wrote about the Cave of the Nymphs in the 13th book of the Odyssey:
"At the head of this harbour there is a large olive tree, and at no great distance
a fine overarching cavern sacred to the nymphs who are called Naiads. There
are mixing bowls within it and wine-jars of stone, and the bees hive there.
Moreover, there are great looms of stone on which the nymphs weave their robes
of sea purple--very curious to see--and at all times there is water within it. It has
two entrances, one facing North by which mortals can go down into the cave,
while the other comes from the South and is more mysterious; mortals cannot
possibly get in by it, it is the way taken by the gods."
The Arlington Tempera contains virtually all of the items in Homer's description. Blake faithfully followed Homer in furnishing his cave. The Naiads use the mixing bowls and stone jars to prepare provisions for the descending souls. On the looms the nymphs weave bodies for them; the purple indicates these bodies contain blood.
Blake loved the looms and used them repeatedly in his prophecies; in his larger prophecies
he described the "nymphs" as vicious wicked women; in fact there are pages of these
wicked women. (The feminine of course connotes the earthly (under the moon), and the
masculine heavenly (under the sun.) (As offensive as this may be to many readers,
I don't know any help for it. It might be considered the guideline that men used in their
subjugation of women. Blake wasn't responsible; he adopted all the ancient symbols,
including this one.)
Blake's picture portrays the two realms, connected by two passages, sometimes called
gates or bars or stairs. The picture shows them as stairs. The prominent gate on the right,
called the northern bar, is especially rich in symbols that Blake used over and over as he wrote, etched, drew and painted.
Immediately to the left of the northern gate is the southern gate of 'return' where worthy mortals ascend into the higher realm of Eternity.
In the upper part of the picture the nymphs prepare souls for the descent into the "sea of time and space". The northern gate is filled with a stream, the current moving downward into the sea. Blake shows two souls scheduled for mortal life; each possesses a tub or pail which the nymphs prepared for them containing spiritual truth and power for the hazardous journey into the world.
At the bottom of the cave one of these 'women' lies in the water blissfully asleep; her tub is turned on its side, all the spiritual things spilled and replaced by the water of mortal life.
The other woman has carefully protected her pail and against the opposition of the nymphs
turned decisively back toward the higher realm; following Heraclitus she may be said to be a dry soul. (This scene evokes Jesus' story of the wise and foolish virgins.)
The dry soul also suggests Thel, who crossed the northern bar, but drew back in horror at the miry clay ahead. The two imaginary humans represent the choices that each of us make every moment: to go the heavenly way or the worldly way, the two ways that Jesus spoke of).
In the symbolic language water denotes matter, the inferior, the worldly. Souls in the higher realm are attracted by the moisture. 'Time and space' is a sea where mortal creatures suffer adventures that may be creative or destructive.
Similar and closely related to dry and moist souls are those awake and those sleeping (this runs like a current throughout the Bible and through Blake as well.)
The River of Adonis in the cave issues into the Sea of Time and Space (one of the common
titles of Blake's tempera). There is (relatively) little to report about the sea; it's just about life, about my life and your life and every brother or sister's life.
But emerging from the sea we find Odysseus, the hero of Homer's Odyssey on the near shore; with his back to the shore he is putting something in the water: in accordance with Leucothea's instructions he is returning her (magic) girdle which she had lent him so he could swim ashore.In the distance Leucothea appears getting her girdle and dissolving "in a spiral of radiant cloud" (Blake and Antiquity page 6).
Behind Odysseus stands his protector goddess, Athena (or Luvah or Jesus) pointing him to the courts above. (The return of Odysseus to his home closely parallels Elijah's ascent on the fiery chariot into Heaven, and of course the Ascension of Our Lord. The thing to remember is that rather than material events these are metaphors. Our metaphors are spacious and temporal; not so in Eternity.)
The upper left of the picture shows God upon a chariot, driven by the four Zoas and surrounded by the immortals. God appears to be a right sleepy god; the import is that it's the inner God who goes to sleep when the soul finds the couch of death and awakens to mortal life (Blake and Antiquity page 15). Raine quotes from The Gates of Paradise:
"My Eternal Man set in Repose
The Female from his darkness rose"
Once you've grasped the whole of this story you may notice how closely it parallels the primary Bible myth of Creation, Fall and eventual Redemption. It's the old, old story, and in the end there's only one story. (Jesus gave us an abbreviated version of it with The Prodigal Son.)
You may find a lot more information about the Arlington Tempera in these posts.