You can find the text of the poem as Blake himself printed it at here. Other than punctuation and capitalization, there are one or two slight wording differences from Bert's version.

RS, Blake's poetry is a little difficult to explain because, as Tim hinted, he did not experience quite the same reality that many of us do. To really understand it, you'd have to study his mystical thought and vision (or delusional system)--something that many people find worthwhile on literary and artistic grounds as well as for his insight, but that I haven't done. This poem is from the Preface to Milton, which according to the notes where I found this poem is about the descent of Milton's soul into Blake's at a crucial time in Blake's life.

But even without that study, I think we can say the poem is basically about the contrast between the Edenic England of Blake's vision and the real industrialized England that he saw as dirty and spiritually impoverished. In the first four lines the images of Jesus seem quite appropriate to the ideal England; in the second four they are highly incongruous. Perhaps the reader is supposed to answer the questions in the first quatrain "yes" and answer those in the second quatrain "no".

The third and fourth quatrains are clearer--he intends to fight industrialization and repression with all the weapons at his command. Both the poem and Blake's life show that the weapons are not physical ones (though he is recorded as having fought with a stick in self-defense); reading the poem as imperialistic or jingoistic would be fatuous. It is radical, though. He describes his goal as the New Jerusalem of the Biblical book of Revelation, that is, as paradise regained. As some of his other poems show, by the way, part of the repression he is fighting is sexual repression--hence the arrows of desire.

And now if Joe Offer is still with me--the Chartist movement was a British movement demanding a "People's Charter" to protect workers' rights. The movement was repressed by law, and the charter was never adopted, of course. The young Marx and Engels were involved in the movement. All I know about it is from Stephen Brust and Emma Bull's historical fantasy Freedom and Necessity. Some of their other novels have folk-music interest, but not that one so much. (I enjoyed it anyway.)