Voltaire may or may not have said that. As far as we know Blake did't say it, but it aptly expressed the feelings of both men. The two had very little else in common.
(As for Voltaire word came that in a visit to England he had been very positively impressed with the Quakers: they didn't have 'hireling priests'.
Blake was born a dissenter; dissenters didn't have priests, but when he came to the age of reason he understood that 'priest' is a generic word embracing all faiths. To propagate (and perhaps live by??) the Law made one a priest in Blake's book. A priest by any name was still a priest, waving the law at people and financially profiting thereby.
Jesus also had problems with priests; the four gospels are full of accounts of that. Eventually the priests killed him.
Perhaps the first thing Blake said about priests is found in Plate 11 of The Marriage of Heaven and Hell:
"The ancient Poets animated all sensible objects with Gods or Geniuses, calling them by the names and adorning them with the properties of woods, rivers, mountains, lakes, cities, nations,and whatever their enlarged & numerous senses could percieve. And particularly they studied the genius of each city & country, placing it under its mental deity. Till a system was formed, which some took advantage of & enslav'd the vulgar by attempting to realize or abstract the mental deities from their objects; thus began Priesthood.
Choosing forms of worship from poetic tales.
And a length they pronounc'd that the Gods had order'd such things.Thus men forgot that All deities reside in the human breast."
Back in Plate 9 we read"As the catterpiller chooses the fairest leaves to lay her eggs on, so the priest lays his curse on the fairest joys. (Erdman 37)
In Plate 5 of Visions of the Daughters of Albion:"The doors of marriage are open, and the Priests in rustling scales Rush into reptile coverts, hiding from the fires of Orc" (Erdman 49)
Or read A Little Boy Lost, Song 50 of Songs of Experience at Erdman 28 (that's a wow!) and on and on it goes. In Jerusalem, plate 69 at Erdman 223:
"Embraces are Comminglings: from the Head even to the Feet: And not a pompous High Priest entering by a Secret Place."
The zoa, Urizen, was the form with which Blake explicated the defiencies of priesthood. The priest of necessity puts himself in place of God, violating the first Commandment: "Thou shalt have no other Gods before me"
"PRELUDIUM TO THE [FIRST] BOOK OF URIZEN
Of the primeval Priests assum'd power,
When Eternals spurn'd back his religion;
And gave him a place in the north,
Obscure, shadowy, void, solitary"(Erdman 70).
Blake hated priests, and he hated what Urizen represented to him in his myth (his System).
The Elect are the priests (discussed in Blake's Milton).
Speaking of the Daughters of Albion:
"the Elect cannot be Redeemed, but Created continually
By Offering & Atonement in the cruelties of Moral Law" (Erdman 98).
Blake organized Mankind into three categories:
The Elect, the Redeemed, and the Reprobate. The Elect were the Priests:
"And the Three Classes of Men.....The first,
the Elect from before the foundation of the World;
The second, the Redeemed.
The Reprobate & form'd to destruction from the Mothers' womb" Erdman 100).