And did those feet in ancient times
Walk upon Englands mountains green:
And was the holy Lamb of God,
On Englands pleasant pastures seen!
And did the Countenance Divine,
Shine forth upon our clouded hills?
And was Jerusalem builded here,
Among these dark Satanic Mills?
Bring me my Bow of burning gold:
Bring me my arrows of desire:
Bring me my Spear: O clouds unfold!
Bring me my Chariot of fire!
I will not cease from Mental Fight,
Nor shall my sword sleep in my hand:
Till we have built Jerusalem,
In Englands green & pleasant Land.
The basis of this famous Blake poem is an apocryphal story of Jesus' unknown years, in which he visits pagan England with Joseph of Arimathea. Conceptually it's linked to the idea of building a second Jerusalem on Earth, the idea of Revelations. Blake doesn't assert truth, but merely wonders if this could have been the case. One phrase that throws many off in the poem is "dark Satanic mills" which seems to be a reference to the changing landscape in the time of the Industrial Revolution.
He resolves to never cease from "Mental Fight" which I take to mean, he seeks to create that paradise, that conceptual Jerusalem. This poem is a well-known anthem within the Anglican church, so my experience with it is somewhat limited. Still, it's fairly easy to understand, I think. It's a resolution to seek paradise through one's own efforts, by force if necessary, though somehow I doubt Blake had a literal Chariot of fire.