Written in 1790-93, I think it is somewhat a poem about revolution. (American Revolution 1776) (French Revolution 1789). It is a response, mainly, to Swedenborg's more fundamentally religious version Heaven and Hell(1758). So, firstly, it is about rebellion and in reversing the dialectic by praising Hell over Heaven and Energy (acting out desire) over Reason. But Blake's philosophy is not so simple, so he doesn't stop there. Because it is a 'marriage of heaven and hell,' he is not simply making an allegory with the devil and rebellion. Blake is calling into question the whole idea of these "contraries," saying that neither should be praised above the other. This can be confusing because we don't know when Blake is being sarcastic and when he's being straight up - and he did this on purpose! This is actually something of a deconstruction because he takes two seeming opposites and challenges the idea that one should ever "top" another.
Religion seeks to end the warfare of contraries because it claims to know a reality beyond existence; Blake wants the warfare to continue because he seeks a reality within existence. (taken from Bloom's article in the link below).
So, in rebellion, Energy is the eternal delight because it is associated with desire, evil and hell. But, in Blake's philosophy, we cannot just embrace one or the other (heaven or hell, reason or energy); we have to embrace both because the life is the play between the two contraries and therefore, neither exists (as Swedenborg's heaven purports to be) above or beyond existence. The contraries need each other. So, energy is the 'eternal' delight but needs reason to give it meaning just as the soul needs the body and as heaven and hell debate about what's good and what's not. For example, rebellion is good at times, but so is loyalty. So, energy is an eternal delight at times, but so is reason. And both, in each pair, challenge each other as if they would attempt to come to some compromise, like the ideal outcome of a political debate.
In the first section, he writes, "Without contraries is no progression."