In the winter of 1417 the papal secretary Poggio Bracciolini made a great discovery. In an abbey in Germany he came across a manuscript of a long-lost classical poem, Lucretius's De Rerum Natura ("On the Nature of the Universe"). This event is vividly described by the renaissance scholar Stephen Greenblatt in The Swerve. He sees it as the origin of the renaissance and, in effect, of modernity.

What was the poem that Poggio rediscovered? Lucretius was a passionate follower of the Greek philosopher Epicurus. He believed that the gods did not concern themselves with mortal affairs and did not create the universe, which was composed of minuscule particles. These atoms move perpetually and randomly through a void. As they do so they "swerve" from a direct course, and may strike against each other. Life is one result of this swerve, as atoms assemble themselves into forms that enable us to see and breathe. At some point our atoms will break free and move on in their eternal course through the void. That meant there was no afterlife, no eternal reward for virtue, and no perpetual punishment for vice. As a result human beings should not fear death. For the short period in which we live and feel desire, pleasure is the only end we should seek. Nothing else matters.

Lucretius created from these philosophical beliefs a poem in the same league as Dante's Divine Comedy and Milton's Paradise Lost. De Rerum Natura contains passionate arguments against the fear of death, as well as some amazing descriptions. Lucretius describes an entirely god-free origin of life, in which living creatures simply heave themselves from the earth, not through the actions of a creator, but as a result of the vital forces of the universe.

It's not hard to imagine why Lucretius was unpopular in the early middle ages. Other pagan philosophers, including Plato and Aristotle, allowed for a creator. Their ethical systems could, with some whittling and squeezing, be fitted to Christian doctrine. Epicureanism, however, simply would not fit. Although it was in fact an austere philosophy in which "pleasure" meant freedom from pain and fear rather than self-indulgence, epicureanism became identified among most Christian writers with swinish self-indulgence. St Jerome even claimed in the fourth century that Lucretius (about whose life we know almost nothing) went mad with love, and then killed himself. Eventually, but unsurprisingly, the great medieval monasteries that preserved classical texts by patient copying of manuscripts came to neglect him.

Most of The Swerve is devoted to this story of loss and retrieval. It begins with a crisis in the Catholic church and in the career of Poggio Bracciolini, who was personal secretary to John XXIII. In the early 15th century there were two popes, one at Rome and the other at Avignon. In 1414 a council was held at Constance in Germany to resolve the dispute. It led to the fall of John XXIII from power. With the end of his master's career, Poggio also lost his job. He set off to scour German monasteries for the classical texts that he was expert at finding and copying. And there he found his Lucretius, which he had copied and sent to his friend Niccolò Niccoli at Florence. Niccoli kept the manuscript for 12 years, and then finally allowed further copies to be produced. From these copies Lucretius found his way into print. With this spread of secularist and atomist thought, Greenblatt argues, the renaissance began.

The story is told with all Greenblatt's style and panache. He brings the silent labours of a medieval scriptorium to life by describing the elaborate sign-language that scribes used to indicate which manuscript they needed to consult: a scribe called for a particularly offensive pagan text such as Lucretius by putting two fingers in his mouth "as if he were gagging". In order to show how the Roman elite valued epicureanism he takes us beneath the ash at Herculaneum into the so-called "House of the Papyri", where fragments of Lucretius and other epicurean writers have been discovered, and uses that setting to evoke the richness of Roman philosophical life. He conveys the passion for texts and for the classical past that drew humanists such as Poggio to scour monastic libraries because he himself shares their fascination with retrieval and discovery.

But is it right to identify the recovery of Lucretius with the beginning of the renaissance? When Poggio found De Rerum Natura, Greenblatt argues, he discovered "a book that would help in time to dismantle his entire world" by bringing a concern for worldly pleasure to the moral life. Greenblatt traces Lucretius's atomism into Galileo's astronomy and Newton's physics. He follows "the swerve" of Lucretius's atoms briefly into the works of Shakespeare and Montaigne. He even sees the focus on pleasure in the Utopia of the devout Catholic Sir Thomas More as resulting from Lucretius. Poggio's discovery, he argues, brought about a liberation for scientific and religious thought that spread throughout Europe.

The story told by the book  epicureanism flourished at Rome, was lost, and then was suddenly rediscovered and transformed the world  reflects the historical outlook of the humanists themselves. It was common for 14th and 15th-century scholars to claim that there was a destruction of classical learning in the middle ages, or, as Greenblatt calls it, "a Great Vanishing", and that they were bringing the classical past back to life. As Francesco Barbaro wrote to Poggio: "You have revived so many illustrious men and such wise men, who were dead from eternity."

Was this story really true? It more or less works for De Rerum Natura, which was indeed "lost" (or at least not often recopied between the 13th and 15th centuries) and then found on a particular day by an individual humanist. But the story that the renaissance suddenly began with a great rediscovery of the pagan past does not work so well in relation to other classical authors. Virgil, Ovid and Aristotle were more or less continuously read from antiquity until the age of print. In many cases humanists found more reliable manuscripts, and they sometimes discovered whole texts. But they did not simply end the "ignorance" of the dark ages. Indeed they tended to exaggerate that ignorance to emphasise their own novelty.

The reason for this is obvious. To have a "renaissance" or rebirth of classical learning, you have to imagine that it died. As well as sharing the humanists' passion for antiquity, Greenblatt shares their prejudice against medieval Christianity, which he portrays with the vividness but also the crudity of a cartoon. "If Lucretius offered a moralised and purified version of the Roman pleasure principle, Christianity offered a moralised and purified version of the Roman pain principle," Greenblatt declares. His descriptions of medieval monasticism emphasise the strict discipline of monastic orders, the erasure of personal identity among scribes and the mortification of the flesh. Greenblatt's version of the middle ages is more or less exactly that of the humanists, in which characterless monks and self-flagellating nuns rejoice in the savage discipline of the church. From this they needed Lucretius to set them free.

Centuries before the rediscovery of Lucretius many Christians incorporated philosophical accounts of pleasure and love from the classical philosophers into their theology. Aquinas and Dante, who play little part in Greenblatt's description of medieval Christianity, found room for both love and pleasure in their philosophies. Those "classical" currents within Catholic thought are a much more likely source than Lucretius for Thomas More's descriptions of the rational pleasures enjoyed by his Utopians. They are among the many strands of thought that lie behind "renaissance" thinking, and indeed behind humanism too.

Greenblatt's story of the unleashing of the pleasure principle on the European world after the discovery of Lucretius conveys his own passion for discovery, and displays his brilliance as a storyteller. The Swerve is, though, a dazzling retelling of the old humanist myth of the heroic liberation of classical learning from centuries of monastic darkness. The light of Rome fades into gloom, sheep graze in the Forum; then the humanists rebel against the orthodoxies of the church, bring about a great recovery of classical texts and generate a new intellectual dawn. This book makes that story into a great read, but it cannot make it entirely true.

This concise, learned and fluently written book tells a remarkable story. It may not quite tell us "how the Renaissance began", as the subtitle rather rashly promises, but the episode it describes is certainly resonant. Highly skilled, close-focus readings of moments of great cultural significance are Stephen Greenblatt's speciality, whether in "new historicist" studies such as Marvellous Possessions, about the European encounter with the New World, or in his more populist biography Will in the World: How Shakespeare became Shakespeare.

The story begins at a monastery in central Germany  almost certainly the Benedictine abbey of Fulda. At its gates, in the first weeks of 1417, arrived an itinerant Florentine scholar by the name of Poggio Bracciolini. A slight, genial man in his mid-30s, he had served as a papal secretary but was currently unemployed owing to the deposition of Pope John XXIII. Today Poggio is best remembered for his vituperative controversy with Lorenzo Valla (a "war of wits" much savoured by Elizabethan comic writers such as Thomas Nashe) and for his salty joke collection, the Facetiae, whose frequent obscenities are rendered in flawless humanist Latin.

But while these later levities give us a feel of the man  droll, engagé, faintly disreputable  his more far-reaching contributions lay elsewhere. A brilliant Latinist and an ardent bibliophile, he was dedicated to recovering the literary and philosophical masterpieces of the classical world, which had been lost and scattered during the so-called dark ages. He was, as Greenblatt puts it, a "book-hunter", and his arrival at Fulda was not as a pilgrim but as an ingenious and tenacious literary detective in search of treasures within the abbey's rich library of manuscripts.

He had already made some important discoveries. A couple of years earlier, in Cluny, he had turned up a codex containing seven orations by Cicero, two of them previously unknown. These works were lost "through the fault of the times", he wrote on his own handwritten copy of the codex, until "by repeated search through the libraries of France and Germany, with the greatest diligence and care, Poggio the Florentine all alone brought them out of the sordid squalor in which they were hidden and back into the light." This dramatic metaphor of imprisonment and rescue is even stronger in Poggio's account of discovering a manuscript of Quintilian's Institutes, "filthy with mould and dust", lying neglected in the monastic library of St Gall, as if in "a sort of foul and gloomy dungeon where not even men convicted of a capital offence would have been stuck away". Here, Greenblatt notes, Poggio's language echoes certain comments he had made a few months earlier, after witnessing the trial and subsequent burning of the Hussite heretic Jerome of Prague.

But it was in Germany in 1417, very probably at Fulda, that he found his biggest prize  a ninth-century manuscript copy containing the entire 7,400-line text of De Rerum Natura ("On the Nature of Things") by Lucretius. This extraordinary philosophical epic poem, composed in Rome around the middle of the first century BC, was at this point known only by name. It was a missing celebrity of the kind Renaissance book-hunters dreamed of finding. Distant hints of its initial impact could be heard in a letter of Cicero's of 54BC, which spoke of its "brilliant genius"; in Ovid's commendation of "the sublime Lucretius"; and in Virgil's lines from the Georgics, "Blessed is he who has succeeded in finding out the causes of things, and has trampled underfoot all fears", the latter phrase echoing a line of Lucretius's  "religion is trampled underfoot"  which would send a shiver through 15th-century Europe.

Of the poet himself, Titus Lucretius Carus (c 99-55BC), almost nothing was known, which remains more or less true today. A brief biographical sketch by St Jerome, written sometime around 400AD, says he killed himself at the age of 44, "after a love-philtre had turned him mad", and that his poetry was written "in the intervals of his insanity". This is undoubtedly, as Greenblatt cautions, a polemical spin by a church father with "an interest in telling cautionary tales about pagan philosophers".

The poem is powerfully ranged against spiritual and supernatural beliefs. It posits a solely material world in which everything is composed of minute particles, the "seeds of the things"  the world of "atoms" previously proposed by Democritus, though Lucretius does not himself use that word. Among the dangerous ideas elaborated from this (as summarised by Greenblatt) are that the universe was not created by divine power; that the soul dies with the body; that there is no afterlife; that all organised religions are superstitious delusions; and that the highest goal of human life is the enhancement of physical happiness in the here and now. In short, it offered a highly charged, poetic exposition of Epicureanism, that philosophy of upbeat fatalism which the church had feared and condemned ever since.

This was the incendiary, anti-religious manifesto, ironically preserved for posterity in a monastery library, which Poggio Bracciolini released anew into the world. Greenblatt pursues with gusto the ramifications of the "Lucretian challenge" in the Renaissance, from the symbolism of Botticelli's La Primavera to the radical cosmology of Giordano Bruno. His title, The Swerve, is itself Lucretian, being a translation of the Latin clinamen, used by Lucretius to describe the unpredictable movements by which particles collide and take on new forms. The rediscovery of Lucretius, it is suggested, was a kind of "swerve" which helped to create the new cultural forms of the Renaissance.

The actual Lucretian codex found by Poggio has since disappeared, as has the direct copy of it done for him by a German scribe, but the beautiful transcript made by his friend Niccolò de' Niccoli survives today; its concluding folio is shown here among the colour plates, a ravishing archival pin-up.

This is a superb essay on the transmission of ideas, but it is also a kind of eulogy to the power and tenacity of manuscripts  a chain of remembrance, a drama of survival. It celebrates the scribal skills of men such as Poggio and Niccolò, whose exquisite, flowing, italic script was the model for early printing fonts designed by Aldus Manutius and others. It also gives insights into the work of those unnamed medieval monks who toiled in the monastic scriptoria, in a spirit of penance as much as of craftmanship, and whose discreetly inserted grumbles  "Thin ink, bad parchment, difficult text"  sometimes remain on the pages they produced.

In 1987, a team headed by a Neapolitan curator and a Norwegian papyrologist succeeded in deciphering and identifying 16 scraps of charred papyrus found at Herculaneum. They proved to be fragments of an early copy of De Rerum Natura  possibly a copy made during the author's lifetime, certainly one in circulation before August AD79, when Herculaneum was engulfed in the eruption of Vesuvius. Without the copyists and book-hunters like Poggio Bracciolini, these tiny textual remnants might have been all that was left of this visionary poem.

Charles Nicholl's Traces Remain: Essays and Explorations will be published by Allen Lane in December