The subtitle of Swerve is "How the World Became Modern," which actually over-promises somewhat. It's actually the story of how Poggio Bracciolini found a manuscript copy of Lucretius' poem, On the Nature of Things and some of the consequences.
To convey the significance of the poem, Greenblatt, who is the John Cogan University Professor of the Humanities at Harvard University, has to explain the significance of the poem, which means explaining something about Epicurus, the rise of Christianity, the world in which Poggio was born and lived, and some of the reactions to and effects of On the Nature of Things. These involve Thomas More, Giordano Bruno, Montaigne, Galileo, even Thomas Jefferson.
We know virtually nothing about Lucretius other than his poem, which he apparently wrote sometime before the birth of Christ (Lucretius, St. Jerome reports, was born 94 BCE). The poem was known in the ancient world. Fragments of it have been found in the ruins of Herculaneum, which was destroyed with Pompeii in 79 CE.
It was not a poem for a Christian—or pagan—world. It denied that the gods (or God) had anything to do with the creation of the world or with human activities in the world. It denied any kind of afterlife, no heaven, no hell. It denied Original Sin and therefore God's forgiveness. It argued that everything—rocks, trees, animals, humans, clouds, stars—absolutely everything is made of invisible particles. These elementary particles are eternal, and while infinite in number they are limited in shape and size. They are in constant motion. Lucretius noted that dust motes in a shaft of sunlight in perfectly still air are moving; we now call this Brownian motion. Everything comes into being as the result of a swerve.
These were not ideas the early Church fathers wanted read and studied and they did their best to stamp them out along with all traces of Epicurus' thought, on which the poem is largely based. Much of the stamping out of ancient books was done by insects, fire, and monks who wanted the vellum for another purpose. Given how much we've lost—only seven of Aeschylus's 80 plays, only seven of Sophocles' 120—it's a miracle that On the Nature of Things survived.
Poggio was humanist and a scribe who became the pope's secretary. When his boss, the pope was disgraced and deposed, he went on a book hunting trip through monasteries in central Germany where he not only found Lucretius' poem in 1417, he had the wit to have it copied and the copy sent home to Florence, where it began to affect the thinking of other humanists. Today, of course, Lucretius' ideas seem almost self-evident; that a Roman poet could have them more than 2,000 years ago seems extraordinary.
Because Greenblatt is comfortable writing about both the early Christian and the Renaissance worlds, Swerve is a lively and fascinating account of the poem, what we know about it, and Poggio Bracciolini who managed being branded a heretic despite his finding and copying a profoundly heretical book, lived to be 79 years old, dying in 1459.