Blake's "Marriage of Heaven and Hell" is generally understood as a polemic arguing that "Hell," which for Blake represents "the active springing from Evil" or the unruly creative force is an essential counterweight to the conservative, rational "heaven." By arguing that hell is coequal with heaven, Blake is challenging religious conservatism. In the same stroke, he challenges the Enlightenment habit of mind that felt that reason could unlock all the mysteries of the universe.
Blake was writing simultaneously to the American and French Revolutions. He lived through a time in which the old established political orders were crashing down. His opposition to religious conservatism and Enlightenment rationality can be plainly seen in the Proverbs of Hell, which includes maxims like the following:
He who desires but acts not, breeds pestilence" and "Every thing possible to be believ'd is an image of truth.
Blake's insistence on the value of creative insight, intuition, and the value of following one's desires is placed in opposition to hard-headed rationality and religious certainty, which he sees as a kind of poison. "As the catterpiller chooses the fairest leaves to lay her eggs on, so the priest lays his curse on the fairest joys," Blake writes. His point is that the energies represented by Hell, while chaotic, can be great forces for good. While they are not commonly recognized as such, they are a needed counterbalance to the "good," "the passive that obeys Reason."