Blake is straight beefing, gang. The Marriage of Heaven and Hell can be understood in a variety of ways, but to get to the heart of it's argument, it really helps to understand that Blake wrote this in response to Swedish philosopher Emmanuel Swedenborg's book Heaven and Hell. In it, Swedenborg basically pulls his best impression of Dante's Divine Comedy, describing what he sees as he wanders around the afterlife in a series of spiritual visions. It's a long, complex book, but it essentially just adds detail to the already-central idea of Christian religion: good folks go to Heaven; bad folks go to Hell.
At first, Blake was super-interested in Swedenborg's visions, since Blake recorded his own visions of God. However, when he read Heaven and Hell, Blake was disappointed—big time. If he were alive today, he probably would have gotten off an epic negative review on Amazon. Instead, Blake came up with The Marriage of Heaven and Hell as a response to Swedenborg's work.
The major issue that he took with Swedenborg is that he essentially just parroted the conventional idea of Heaven and Hell. In the book, Blake writes that "Swedenborg boasts that what he writes is new; though it is only the contents or index of already published books" (7.14). Blake's major problem with this is that he believed that human beings possessed both good and evil inside them—and that was a good thing. To Blake, this conflict made humans… well, human.
He also saw these qualities less in terms of good and bad, and more in terms of restraint and impulse. And Blake was big on impulses. So, Blake's view of human nature is not the separate Heaven and Hell that Swedenborg describes. Instead, it's a marriage of the two.
And Blake doesn't miss a chance to bust on Swedie for getting this wrong. He compares Swedenborg's writings to "the linen clothes folded up" outside Jesus's tomb (1.23). In other words, they're useless. Later in the book, he compares Swedenborg to a man who thinks he's smart, just because he's smarter than his pet monkey. Then he stops beating around the bush and states his case directly: "Swedenborg's writings are a recapitulation of all superficial opinions, and an analysis of the more sublime, but no further" (7.18).
To Blake, Swedenborg was like a man "who only holds a candle in the sunshine" (7.20), not bringing anything new to the table. He was also a symbol of conventional religious views. Blake was anything but conventional, and so Swedenborg became the perfect target for his broader criticisms of the good-bad, either-or way we tend to view the human spirit.