Actually, read in context Blake’s preface to his work Milton (And did those feet in ancient times) appropriated by Parry for his jingoistic Jerusalem is a call to arms for the creative against the people who can only see materialism.
Rather than simply retelling the myth of Joseph of Arimathea bringing a young Jesus to England, it questions the insular view that England was so amazing that the lamb of god would up sticks from the Middle East to visit some hills in England (note the punctuation including exclMation marks and question marks in the original text)
It then moves on to challenge the view that there is anything holy In Blake’s contemporary England - again Blake questions this by asking whether Jesus can be present among “these (not those) dark satanic mills”, whether these mills are flour mills, cotton mills or churches (ie the established religion) is open to debate. Personally (and person interpretation is very much par for the course with Blake) I think he’s talking about the tangible status quo, where money and power sit.
He then talks about the future. The past is a lie, the present is shit, so what are we going to do? Can we make it better? Yes we can if we reject the status quo, we need to fight to take up arms of bows, arrows, spears and swords, not physically but mentally, we need to fight for change, to bring a new era where our God is not money and power but something more pure.
Whether the reference to “chariots of fire” relates to the Egyptian sun god Ra riding his chariot across the sky or the appropriation of the imagery in several places in the Bible, I think it’s asking us to see something new (or indeed old and largely forgotten or hidden- at Blake’s time). In kings 6:17 Elisha asks God to open a persons eyes, whereupon they see horses and chariots of fire, the army who threaten to overpower them have their eyes closed and do not see the Amazing truth that those with their eyes truely open see. This enables the army (a symbol of power and establishment) to be crushed
Just my thoughts on what Blake might have meant. Others might interpret it differently though (that’s the beauty of Blake).
But one things for sure - it’s not - isn’t England great? So probably not suitable for a national anthem!