Stephen Greenblatt wrote a fascinating book The Swerve about the rediscovery in 1417 of a work of philosophy from antiquity. The Nature of Things was written by Lucretius a few years before the birth of Christ. It provides an account of the world profoundly at odds with religion - atomism and epicureanism. Stephen's thesis is that over time the implications of this work changed the course of history.
We start the story at the Council of Constance. Christendom has three popes and is figuring out how to reduce this to a more manageable number. An employee of one of the losing popes, one Poggio Bracciolini, now has time on his hands and spends it hunting for old manuscripts in remote monasteries . . .