The Perennial Philosophy was about as good a description of Blake's thought as we can find. Not trained and conditioned to the Enlightenment philosophy of radical materialism, he used for it his favorite metaphor, Bacon, Newton, and Locke (Jerusalem Plate 54, line 22; Erdman 203).
Blake was steeped with the wisdom of the ages, but it was in poor repute to cultured people of his day. At its apogee the radical materialists became logical positivists; they considered statements of anything not measureable in height or weight to be meaningless; they frowned on emotion and considered it enthusiasm (a pejorative).
The cultured people of that day tended to be primarily deistical and purely materialistic: God created the world and wound it up like a clock and thereafter had no interest in what went on in this planet.
Blake had his problems with God, but he knew that the word indicated something other than that; he despised the Deists. In Plate 52 at the beginning of his Chapter Three of Jerusalem (To the Deists) he told them what's what. Read it, and get a fuller grasp of Blake's faith.
The traditional symbols make up the stuff of poetry; 18th century English poetry by and large can be seen as peurile. Blake initiated the Romantic movement in poetry leading to the Romantic poets and artists; they were rich in the symbolism of tradition.
You may trace the deterioration of 18th Century English poetic thought through Shakespeare, Spencer, and Milton to William Hayley. At that low ebb Blake and the other Romantics intervened to bring about another renaissance.
Society had been utterly commercial, especially in things like marriage, even in Dickens' day. (The first 50 pages of his masterpiece, Our Mutual Friend, show to what lengths they went even in his day.)
Blake revolted emphatically against materialistic attitudes, disgusting the materialists who made up his age. So he spent his creative life in obscurity, his brilliance largely unrecognized. (There are others like that, the general consequence of caring more for art than money.)
He was excluded because his thought forms were too radical; his language lacked the comfortable familiarity of the conventional majority. More than that he was too deep: completely beyond comprehension to the positivists, and just too dense for everyone. They called him mad!
Until today! Blake is coming alive in the 21st Century. Throughout the last century you could find innumerable epigrams by Blake in books and chapters of books. And now there's a rebirth of reading, critique, commentaries, etc. (This blog is an example.)