When I came home, on the abyss of the five senses, where a flat-sided steep frowns over the present world, I saw a mighty Devil folded in black clouds hovering on the sides of the rock; with corroding fires he wrote the following sentence now perceived by the minds of men, and read by them on earth:— (3.2)
These "corroding fires" seem pretty appropriate for a devil to write with, but they're also a sly reference to the copper engraving technique that William Blake pioneered for his own work. Here, Blake's really referring to himself as a "might Devil." We wonder if that was a favorite nickname that his friends called him.
The ancient poets animated all sensible objects with Gods or Geniuses, calling them by the names and adorning them with properties of woods, rivers, mountains, lakes, cities, nations, and whatever their enlarged and numerous senses could perceive. (4.71)
Before there was religion, Blake claims, we had poetry. Poets were the ones responsible for naming the world and everything in it, because only they had the ability to appreciate the world as a whole. Here, we get a bit more of Blake patting himself on the back for being a poet. Good job, Willy.
Isaiah answered: "I saw no God, nor heard any, in a finite organical perception: but my senses discovered the infinite in everything; and as I was then persuaded, and remained confirmed, that the voice of honest indignation is the voice of God, I cared not for consequences, but wrote." (5.2)
It's hard not to see Blake referring to himself here again. Isaiah explains that the power of his perceptions convinced him to write his prophecies down—regardless of the consequences for doing so. It's not hard to think of Blake making the same claim about his own work, which was pretty… you know, out there.
We of Israel taught that the Poetic Genius (as you now call it) was the first principle, and all the others merely derivative, which was the cause of our despising the Priests and Philosophers of other countries, and prophesying that all Gods would at last be proved to originate in ours, and to be the tributaries of the Poetic Genius. (5.6)
Have you ever wondered where religion comes from? Blake's answer is: poetry. In fact, the poets had it right in the first place. The priests and philosophers were just riffing on the poets' original ideas ("Poetic Genius"). Unsurprisingly, Blake thinks that religion gets a lot of the poets' ideas wrong.
I was in a printing-house in Hell, and saw the method in which knowledge is transmitted from generation to generation. (6.1)
Of all the places to see in Hell, a publishing house is a pretty weird tourist destination. However, the printing-house was where Blake worked his day job. Not only did he know a lot about what went into printing, he also had an important perspective on what printing actually did for the world. Namely, as he describes here, printing was how ideas and knowledge got around—you know, before the Internet. Since he printed The Marriage of Heaven and Hell, himself, Blake must have been particularly interested in how his ideas would get out into the world. We wonder what he would have thought of us.