John Milton's Christian epic – or at any rate most of it – came out in 1667. Extremely ambitious in design and scope (yet a slimmish fast read compared with some contemporary offerings), it puzzled and troubled readers at least as much as it ravished them. The printer, halfway through the first run, persuaded Milton to write little plot summaries before each book to help orient the reader, and to give "a reason of that which stumbled many others, why the Poem rimes not". Milton, a bit tetchily, obliged – of which more later. Nor were these difficulties the only ones. His poet-contemporary, Andrew Marvell, wrote:

"The Argument

Held me a while misdoubting his Intent

That he would ruine (for I saw him strong)

The sacred truths to Fable and old Song."

If we take him straight, Marvell is worrying that Milton might inadvertently tumble the entire Christian edifice of fall and redemption into just another myth – because he was such a good epic poet. He compares him to Samson pulling down the temple "to revenge his sight" – a tellingly personal comparison, given that Milton, also, had become blind (and was very interested in Samson). Marvell is saying all this in a poem of compliment, so even his worries are designed to flatter; but that doesn't mean they are not real worries. He spends a good third of his poem finding different extreme ways to describe the potentially self-defeating nature of Milton's project.

Marvell was one of the first to express this unease in print; he was certainly not the last. We find versions of it all over the reception of Milton's poem. In the responses, for example, of Blake, or Shelley, who saw in Milton an unconscious defence of Satan; in the hostile reaction of William Empson to Milton's depiction of God; in the lively deliberate humanist borrowing of Philip Pullman. All these, and many more, see in Paradise Lost a vast, trickily hybrid production which does something other than it says on the tin. They don't all see the problem to lie in the same place, and most of them conclude that Milton was doing the world some sort of service anyway. But that Milton's eye was (at least) double they agree. Frequently they decide that Milton failed – but only just – to write into the poem the explicit critique of its theodicy which they personally held and would now go on to explain better. Poets themselves for the most part, they nevertheless betray from time to time the wishful hint that if Milton had stuck to the "cool element") of prose his theological position would have been a bit easier to nail.

Yet Milton's intention to write a great English poem was primary. He had been thinking about it for more than 20 years. Some time in 1641-42 he jotted down some ideas as to what his poem should be about. These were drawn from the Bible, both the Old and the New Testaments, but also ranged across national themes. He flirted, for example, with writing on King Arthur. His drive to write was both religious and moral in the pattern of the Renaissance humanist. "The end of learning," he wrote at about this time, "is to repair the ruins of our first parents by regaining to know God aright." In other words, you rebuilt from the destruction wrought by the fall through study. In saying this Milton conceived of everything he read (and his range was both wide and deep) as tending to this end: Plato to Euripides to Homer to Ovid (a particular favourite) to Ariosto and Tasso and Shakespeare as well as Job and the Revelation of St John the Divine. Everything out there of value could help to redeem you. For Milton, all knowledge led you, though in a winding route and on a "dark voyage", back to paradise, walking and talking with God in the cool of the evening. It is no wonder, then (though as we have seen Marvell did wonder) that his paradise contained as much as he could stuff in of everything he had learned, from everywhere.

But Milton in 1641 was clear that he couldn't get around to it yet. He must put aside his poetic ambition in order to serve his country's political needs in prose writing. There was a revolution to sort out. Not until after the collapse of the Commonwealth (and following his own imprisonment in 1659) did Milton turn the whole of his attention to the poetry he had been planning for so long. Freed from his civic task by comprehensive political defeat, reoriented to the tendency of humankind to ruin its dwelling place and disappoint its own hopes, Milton was at last ready to begin.