'Jerusalem' would be unknown today had it not been set to music by Hubert Parry. It is Parry's heart-stirring music which made 'Jerusalem' the anthem that it is.

The poem (hymn) starts with four rhetorical questions... 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? The answer to all four questions is "No". No, Jesus Christ did not ever set foot in England (as far as we know). No, Jerusalem (the Holy City) was not built "among these dark Satanic Mills". The mills are symbols of the evils of the Industrial Age. The poem ends with a personal commitment to devote one's life to building Jerusalem (paradise on earth) in England. I will not cease from Mental Fight, Nor shall my Sword sleep in my hand: Till we have built Jerusalem, In England's green & pleasant Land. From the Wikipedia article (http://en.wikipedia.org/wiki/And_did_those_feet_in_ancient_time#Revolution) Several of Blake's poems and paintings express a notion of universal humanity: "As all men are alike (tho' infinitely various)". He retained an active interest in social and political events for all his life, but was often forced to resort to cloaking social idealism and political statements in Protestant mystical allegory. Even though the poem was written during the Napoleonic Wars, Blake was an outspoken supporter of the French Revolution, and Napoleon claimed to be continuing this revolution.[22] The poem expressed his desire for radical change without overt sedition. I would conjecture that one reason that the song has such wide popularity is that it doesn't actually specify the characteristics that "Jerusalem" would have and thus conservatives, liberals and socialists can all sing this song even if they have radically different, even opposing concepts of the Jerusalem that they want to build in England.

http://en.wikipedia.org/wiki/And_did_those_feet_in_ancient_time#Use_as_a_hymn I wish I had known without checking Wikipedia. From the article, it looks like it gained popularity as a hymn in church services on St George's Day (23 April), the patron saint of England.