Blake’s childhood in London was relatively poor. He was happy and normal – apart from occasional visions of angels and prophetic beings in his everyday surroundings. He went to a local school till he was ten then to a drawing academy for five years; after that, he was apprenticed for seven years to an engraver. He spent a period as a student at the Royal Academy, was employed as a commercial engraver, and eventually set up his own business in 1784.

Having been brought up in a Christian household, he can be linked to at least two definable religious movements (Moravian and Swedenborgian) which made up part of London's spiritual life and which influenced his later thinking (http://www.blackwell-compass.com/subject/literature/article_view?article_id=lico_articles_bpl370 ). Blake has been hailed as a humanist thinker, but his beliefs were confused: he was a devout follower of Jesus Christ who evidently distrusted the potential for misuse of organised religion.

And priests in black gowns were walking their rounds
and binding with briars my joys and desires. (Songs of Experience)

As the catterpiller chooses the fairest leaves to lay her eggs on, so the priest lays his curse on the fairest joys. (The Marriage of Heaven and Hell)

The reason Milton wrote in fetters when he wrote of Angels & God, and at liberty when of Devils & Hell, is because he was a true Poet and of the Devil’s party without knowing it. (The Marriage of Heaven and Hell)

Blake engraved the long poem “Milton” between 1804 and 1808 so he was 51 when it was completed. Jerusalem is part of the Preface. The concept of “humanism” which had been developing in Europe since the 14th century focused on Roman and Greek ideals, but the Preface makes it clear that Blake’s ideal was nothing to do with the “Stolen and Perverted Writings of Homer & Ovid, of Plato & Cicero, which all Men ought to contemn. … We do not want either Greek or Roman Models if we are but just and true to our own Imaginations, those Worlds of Eternity in which we shall live forever in Jesus our Lord.”
Both the poem and those lines of the Preface which immediately precede it make it quite clear that the first stanza of Jerusalem is about Jesus. The idea of Jesus being in England comes from an apocryphal story that “in ancient time” He came to the town of Glastonbury, accompanied by Joseph of Arimathea. The references to Jerusalem would have been understood by Blake's contemporaries as yearnings for an ideal state, possibly even in the afterlife; they do not refer to the city in modern Israel, but to the “New Jerusalem” of the New Testament Book of Revelation. This itself is a poetic and visionary book, capable of a variety of confused and confusing interpretations.

Blake melded many Biblical references into his own personal mythology. This mythology is highly confusing and at times appears more mad than mystical, but the references he makes to both New and Old Testament names would have been instantly recognisable to his readers of the time, setting up mental associations of which our more secular age is ignorant. His preference was for the angry, warlike, avenging God of the Prophets, with strong emphasis on right and wrong, reward and punishment. Hence his references, as in Jerusalem, to chariots, fire, arrows and spears. Curiously he does not seem at all inclined to have his characters use gunpowder despite the British armaments produced by the "dark Satanic mills" being the best in the world at the time.

Blake was enthusiastic about the American Revolution of 1775 and the 1789 French Revolution, and he wrote long poems about the youthful energy of the revolutionary spirit, but the excesses of the subsequent French Reign of Terror disgusted him.

He was always a radical thinker who was deeply moved by the poverty he saw daily around him and which he was often in danger of sharing. He distrusted the abuse and misapplication of power in conventional politics. In his writing there is frequently a sense of criticism of England for not doing the best it could for its people. It is that criticism which appears to me to underlie Jerusalem.

As a poet he did not conform to the literary conventions of his day, and his combination of prophetic poetry with his own illustrations was highly novel. He was an advocate of free love and free thought: “The Imagination which Liveth for Ever”. What Mrs Blake thought of all this is not sure. Although she helped him to bind his engraved volumes of poetry, she is also recorded as having said, “I have very little of Mr. Blake's company. He is always in Paradise.”

He died childless in 1827, and in such poverty that he was buried in a shared grave paid for with borrowed money. He left behind an extraordinary legacy of paintings, engravings and poetry.