Two links tell this whole story: my Blake's Sources and an address by Kathleen Raine. She tells of being at Cambridge in 1927; Keynes had just published, making Blake's Works more readily available, and everyone was reading Blake. In Raine's address she explicated how Blake (heretofore considered a rustic genius) was fully cognizant of the the 'perennial philosophy', while the materialistic culture of his days (and ours!) were completely ignorant of it.
Blake felt that the Enlightenment was not what it was cracked up to be: The Industrial Revolution had meant disaster for millions of Brits who had lost their right to use public grazing lands; they had flooded into London and other cities in search of employment (incidentally a similar migration is going on today throughout the world). They got starvation jobs or nothing and slowly starved. The upper classes rejoiced at finding such cheap labor (just as ours do today!)
Bacon, Newton, and Locke had set the tone of intellectual culture in Blake's day-- a tone of pure materialism, an absence of spiritual values (the same tone we live in today!) (In 1990 I told a casual acquaintance that the only hope for America was a spiritual revolution; he said No! a mental revolution; both statements were and are equally true.) The absence of spiritual values was attested by the corruption and depravity of the religious establishment (then and now!); this led the youthful Blake to make his disparaging remark about priesthood in Plate 11 of MHH.
Blake confessed his intellectual interests and the sources that fed his spirit in the letter he wrote to Flaxman in 1800:
"Now my lot in the Heavens is this; Milton lovd me in childhood & shewd me his face Ezra came with Isaiah the Prophet, but Shakespeare in riper years gave me his hand Paracelsus & Behmen appeard to me. terrors appeard in the Heavens above"
These authorities were not often used by Bacon,
Newton, or Locke. Neither are they used by the
intellectual paragons of our day. Did you ever hear of