Prophets

Blake loved to talk about prophecy, so much so that sometimes his longer works are referred to as "the prophetic books" (true story). Anyway, the speaker talks about Jesus in this poem (he's a prophet), and then there's a reference to Elijah (another prophet), and then, well, there's the speaker himself. He identifies himself with Elijah (the original rider of a chariot of fire), and implies that he's as special as Jesus and Elijah and has been singled out for a greater purpose. What purpose is that? To spearhead the building of Jerusalem, of course, eliminating the "dark Satanic mills" along the way.

• Lines 1-2: This is the speaker's first reference to Jesus, but he doesn't call him Jesus (actually, he doesn't call him Jesus anywhere in the poem). Instead, he just drops some synechdoche ("those feet" stands in for Jesus's whole body) and leaves it at that. The cryptic reference to the most famous prophet of all foreshadows the poem's later prophetic themes.
• Lines 3-4: Again, the speaker refers to Jesus, but by one of his nicknames: Lamb of God. Note: this is not a reference to these guys, but a reference to a biblical phrase that occurs many times. The reference tells us that the speaker is talking about Jesus and also suggests that there may be some sacrifices ahead in the near future. 
• Lines 11-12: The "Chariot of Fire" is an allusion to 2 Kings, where the prophet Elijah is taken up to Heaven in a chariot of fire. The speaker is clearly identifying himself with a famous biblical prophet.
• Lines 13-16: The speaker's claim that he will not "cease from Mental Fight" until Jerusalem is built again has a prophetic air about it. The speaker is almost predicting that Jerusalem will be built again, in the same way a prophet would. The "Mental Fight," the sword not sleeping, and the building of Jerusalem are all metaphors for changing the world, for healing all of England's division, pain, and disharmony.