The speaker of "Jerusalem" looks forward to a new era, but also backwards to an "ancient time." He wonders if Jesus ever visited England, and if Jerusalem was once built there. While the speaker isn't quite sure if this ever actually happened (the "past" he imagines is more legend than anything), it's almost like he wants to imagine that it did. That way he can imagine a future that repeats the past (if you can follow that logic). The past the speaker talks about is peaceful, holy, and magical proof that England's "clouded hills" were once full of divine light and grace. Far out.
The poem is made up of a lot of questions, like… a lot. This suggests that we can never really truly know the past, even though we may think we do.
The past is important for the speaker because it's a model or template… or blueprint for the future. Jerusalem was perhaps already built once, and can be built again.
In "Jerusalem," the speaker is obsessed with a potentially legendary past, when Jesus visited England. Man, those were the days. All this reflection on that past gets him thinking that things really need to change in England. The New Jerusalem, the speaker's symbol of a reborn, holy, pure, and better England, must be built again. In other words, the time is ripe for revolution, only it doesn't have to be a huge, bloody mess like the 1789 French Revolution. While the speaker doesn't go into a whole lot of detail about just how to bring the changes that are necessary, he does clearly state that England is full of "Satanic mills" (evil, dirty factories and other bad things) and some weaponry will be necessary (a bow and arrow, for example). At the same time, he does also imply that some type of mental action will be necessary to bring about the changes he thinks are necessary.
Change must come to England, no doubt about it. Even though the speaker talks about weapons, the change can be as simple as looking at things differently.
When things get so bad that the only way to describe them is with the word "Satanic," something must be done. Everything must be completely burned away, destroyed, and remade.
"Jerusalem" is a poem about perseverance if there ever was one. Think about it. In the poem's third and fourth stanzas, the speaker is essentially getting ready for battle (bring me my bow, bring me my spear, etc.). The goal is to build Jerusalem, and the speaker isn't going to stop until that happens. He wants to believe that it's already been built once before (in England), so at least there's some hope. Nevertheless, it's not an easy task, and the speaker flat out says that his word will not sleep, and he will not cease, until that task is done.
Perseverance is about keeping your eye on the prize, keeping your sword sharp, your arrows ready, and your chariot lit and moving—metaphorically speaking of course.
Perseverance in pursuit of a goal is just like a battle. That's why the speaker is all about those weapons.