And did those feet in ancient time
Walk upon England's mountains green?
And was the holy Lamb of God
On England's 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!
The juxtaposition of this line between “divine countenance” and the “satanic mills” signifies Jerusalem’s history as a human construction, an ideological battle field (he later calls it a “mental fight”). It was fought over between empires for centuries and still is today. 
 This is linked to the Book Of Revelation (3:12 and 21:2) describing a Second Coming, wherein Jesus establishes a New Jerusalem. The Christian church in general, and the English Church in particular, has long used Jerusalem as a metaphor for Heaven, a place of universal love and peace.
And Did Those Feet In Ancient Time