There are two key concepts that you need to be aware of in order to fully understand and comprehend what Blake is trying to say and achieve in this work. Firstly, and this is a concept that is explored in Blake's other major poetic work, Songs of Innocence and Experience, Blake saw life as consisting in a series of contrary forces that make up the world. These two major dominant forces are characterised in this work as good and evil, which are equated with heaven and hell. Blake firmly believed that any system or thought that tried to ignore on half of such a dichotomy was doomed to failure, as both elements of the dichotomy are necessary in order to recognise the complex nature of human experience.
Secondly, and stemming from the idea of dichotomies above, Blake was harshly critical of the church of his day, that he saw as focusing so much on the good of man alone that it actually became a tyrannical force that was used to enslave and brainwash Christians of the day. He believed that any church that allowed children to be sold into conditions that resembled slavery through child labour and encouraged people not to question such events was profoundly wrong and tyrannical. This is the kind of dangerous excess that can result from a failure to recognise the explicit union of the two contrary states of heaven and hell.