Ingrid Rowland in The New York Review of Books:
On All Hallow’s Eve of 1517, Martin Luther, Augustinian friar and professor of theology, posted a broadsheet on the faculty bulletin board of tiny, provincial Wittenberg University in the German state of Saxony (which happened to be the door of the church attached to the local lord’s castle). The poster was no Halloween prank; it proclaimed, according to academic custom, his willingness to debate a series of propositions in public. Although he also sent copies of the same broadsheet to important statesmen, churchmen, and academics outside Wittenberg, no one seems to have taken up his challenge to a formal discussion. His propositions were too explosive for that; in blunt, forceful language, they questioned the basic beliefs of the church to which, as a Hermit of Saint Augustine, he had vowed his obedience.
Luther would say that his life’s turning point came two years later, when he had a sudden revelation about the nature of Christian salvation. For his contemporaries, however, the posting of his ninety-five theses in 1517 set off the spark that ignited the Protestant Reformation, and the Reformation in turn marked a fundamental stage in the forging of a collective German identity.