During the three years that the Blakes lived at Felpham, (out of the city of London or its outskirts for the only time in their lives), he was writing Milton and Jerusalem as well as working on more trivial projects dreamed up by his sponsor and 'corporeal' friend Hayley. In this letter to Thomas Butts he addresses the difficulty in writing to a level of understanding that is opaque to those who haven't the intellect or spiritual development to receive it. Jesus taught in parables which were meant to teach spiritual truths which were opaque to those without spiritual discernment. Blake wants to lay bare his understanding of cosmic events as spiritual teaching which can't be perceived without receptivity through the spirit. He recognizes God as the source of the truth which has come to him and which he has written in his poetry. He knows that people like Hayley who care nothing about nurturing a relationship with God will turn away from his writing. But if the parables of Jesus or the poetry of Blake can make a little crack in that wall which men erect to hide from God, the seed may be planted, germinate, flourish, blossom and eventually bear fruit.
Letter 27, To Thomas Butts, Felpham' July 6. 1803, (E 730):
Thus I hope that all our three years trouble Ends in Good Luck at last & shall be forgot by my affections & only rememberd by my Understanding to be a Memento in time to come & to speak to future generations by a Sublime Allegory which is now perfectly completed into a Grand Poem[.] I may praise it since I dare not pretend to be any other than the Secretary the Authors are in Eternity I consider it as the Grandest Poem that This World Contains. Allegory addressd to the Intellectual powers while it is altogether hidden from the Corporeal Understanding is My Definition of the Most Sublime Poetry. it is also somewhat in the same manner defind by Plato. This Poem shall by Divine Assistance be progressively Printed & Ornamented with Prints & given to the Public--But of this work I take care to say little to Mr H. since he is as much averse to my poetry as he is to a Chapter in the Bible"
When we find the poetry difficult to understand, we may also remember what Blake wrote to Dr. Truxler (E 702) concerning the way wise writers seek to get a response from their readers.
"The wisest of the Ancients considerd what is not too Explicit as the fittest for Instruction because it rouzes the faculties to act."
'not too Explicit'