1517 - Protestant Reformation
The Protestant Reformation begins on 31st October 1517 in Wittenberg (modern day Saxony-Anhalt in eastern Germany) when Martin Luther nails his "Ninety-Five Theses on the Power and Efficacy of Indulgences" to the door of Castle Church (officially known as All Saints Church), in Wittenberg. The Reformation leads to a religious schism in Europe - broadly speaking northern Europe becomes Protestant while southern Europe remains Catholic.
In the Holy Roman Empire, the ruling Habsburgs remain fiercely Catholic, but individual territories convert to Protestantism. Former Alemannic lands in Germany and Alsace remain largely Catholic, except for the Margraviate of Baden-Durlach. The former Alemannic lands in Switzerland largely convert to Protestantism, except for the central Swiss Alps cantons that formed the original Old Swiss Confederacy.
The Reformation generates long term religious tensions within the Holy Roman Empire that occasionally spill over into armed conflict, culminating in the Thirty Years' War of 1618-1648.
Image Credit: By User:Cethegus (Self-photographed) [Public domain], via Wikimedia Commons