Reformation - Diarmaid MacCulloch

The Reformation, which began in 1519 when Martin Luther famously nailed his 95 Theses to the door of Wittenberg Cathedral, is the seismic event in European history over the past 1000 years. The medieval world was torn apart, and not just European religion, but thought, culture, society, state systems, personal relations - everything - was turned upside down. Just about everything which followed in European history - and because of the Pilgrim Fathers, in America too - can be traced back in one way or another to the Reformation and the Counter-Reformation which it provoked.
This is where the modern world painfully and dramatically began.