Blake was a highly symbolic poet (and painter); to understand much of his thought requires acquaintance with a body of symbols that go back to the dawn of civilization, and up to the 19th century. In an age when only the material seemed to matter Blake was (and continues to be) highly opaque to the pure materialist. Such a person will find most of Blake's ideas meaningless.
But at the deepest level his ideas are the veritable stuff of life: love and hate, good and evil, life and death, and many ideas with urgent meaning. A high proportion of people prefer to turn aside from these questions, but you can be sure that their unconscious is full of them.
Above all Blake is about matter and spirit, at the great dividing line: do you see yourself primarily as a body or as spirit?
Begin with the conclusion, to be supported by an overwhelming body of evidence stretching from Heraclitus in the 6th century BC to the present:
Our mortal life is a vale of tears to which we have lapsed from Eternity and from which we will (may?) eventually turn back into the Higher Realm. This myth conforms very closely to the Gnostics, the Platonists, and of course most of Eastern Religion. In the Christian tradition one can find vestiges of it in many of the mystics, notably Meister Eckhart, in Mexican folk culture and in fact universally.
The western mind revolts from this "never-never land" at least on the conscious level, and many readers may dismiss Blake's myth as not worth their attention. But Freud, Jung, and many other psychologists found strong evidence for it in the unconscious.
The select few who have not succumbed to the prevailing radical materialism may rightfully expect an entirely new world of grace and enchantment to open before their minds. The biblically oriented may perceive that all Blake's poetic and artistic work fits into a scheme of cosmic/psychic meaning; closely following the Bible it describes the pattern of Paradise, the Fall, a gradual redemption, and the final Rapture.
This material comes from Chapter Ten of Blake's Primer.