In the 17th Century George Fox (the first Quaker) interrupted 'divine services' to rail against Steeple Houses and Hireling Priests. Writing in the 18th and 19th centuries Blake must have been aware of Fox's famous words (although we have no record that he ever mentioned Fox).
He must also have been fully aware of the French description of Priests (attributed to Voltaire): "The first priest was the first knave who met the first fool". We have every reason for Blake to believe that priests were knaves. Look at this passage in the Marriage of Heaven and Hell (Plate 9):
"When thou seest an Eagle, thou seest a portion of Genius. lift up thy head!
As the catterpiller chooses the fairest leaves to lay her eggs on, so the priest lays his curse on the fairest joys."
We may expect to see (or read) Blake's denunciation of priests throughout his works; look at Visions of the Daughters of Albion (Erdman 47):
"With what sense does the parson claim the labour
of the farmer? What are his nets & gins and
traps; & how does he surround him With cold
floods of abstraction, and with forests of
solitude, To build him castles and high spires,
where kings and priests may dwell;".
In his Annotations on Bishop Watson's Apology for the Bible (E615) Blake wrote:
"The Bible or Word of God, Exclusive of Conscience or the Word of God Universal, is that Abomination which like the Jewish ceremonies is for ever removed & henceforth every man may\ converse with God & be a King & Priest in his own house".
Here's an interesting poem in Songs of Experience:
The GARDEN of LOVE
I went to the Garden of Love,
And saw what I never had seen:
A Chapel was built in the midst,
Where I used to play on the green.
And the gates of this Chapel were shut,
And Thou shalt not. writ over the door;
So I turn'd to the Garden of Love,
That so many sweet flowers bore
And I saw it was filled with graves,
And tomb-stones where flowers should be:
And Priests in black gowns, were walking their rounds,
And binding with briars, my joys & desires.