Ladies and gentlemen, the man, the myth, the legend, the… wait, that's not right. The legend, the myth, the man? Nope, that's not right either. Wait, here it is: a man who's into myths and legends. That's the one we're looking for because, well, that's a great way to describe the speaker of this poem. The first half of the poem, you will recall, explores the possibility that Jesus once visited England nearly 1800 years before the poem's composition. What is more, the speaker even ponders whether Jerusalem itself (Blake's symbol for the most peaceful, holy, perfect place imaginable) was once built in England. In Blake's heyday, the whole Jesus-visited-England thing was sort of an urban legend, a popular myth or superstition. While the speaker doesn't come right out and endorse it, he's definitely willing to entertain the idea.
But why? The speaker also clearly has a social conscience. That little jab at the "dark Satanic mills" is a not-so-secret critique of the Industrial Revolution and all the filthy, dirty, horrible things that came along with it (child labor, poor working conditions, death—all sorts of other charming stuff). If Jesus once visited England, and shined some divine light on England's "cloudy hills," well then there's hope for the future (he may even come again). Rather than sit around and wait for Jesus (the Lamb of God) to come back, the speaker figures he might as well get the revolution started. He asks for his bow and arrow, and his spear, and his chariot of fire (it's all very biblical, folks), and while he doesn't say so directly, he's really asking for these weapons so he can start laying waste to all the bad things around him so Jerusalem can be built again (and eternal peace can reign supreme).
Well…. yeah, sort of. You see, the speaker is committed to a social revolution (remember, he says his sword will not sleep in his hand), but a revolution that isn't violent (forget the weapons for a minute). His refusal to cease from "Mental Fight" is really important because it balances out all the violence associated with weapons. The weapons are mostly symbolic, and the real war will be a war of ideas—a mental war in which changing one's perspective matters more than anything else (think about things like contemporary racism, and how the emphasis is on acceptance and tolerance, not taking swords and spears and attacking racists).
A God-fearing man, committed to social revolution, willing to entertain semi-mythical ideas, and fired up like there's no tomorrow? Hmm—there was once a real person who was pretty much exactly the same as the speaker of this poem, and his name was—wait for it—William Blake. Okay, okay, so we know technically the speaker and the poet are two different people, but with Blake, the rules can be broken a little bit (Blake would approve, since he was all about breaking down barriers and "melting apparent surfaces away").
Blake was a radical if there ever was one, and he was committed to bringing about the New Jerusalem by working to end things like poor working conditions, poverty, and just about everything else that he thought was simply no good. The guy wrote poems about the horrible conditions chimney sweepers dealt with, for example, so you can't go wrong by assuming that the speaker is, at the very least, Blake's close relative. Read more about his revolutionary politics here and here.