Christian History has posted a helpful background paper by Dt. Eric W. Gritsch on the 1517 event of Luther Posting the 95 Theses: "An obscure monk invited debate on a pressing church issue—and touched off a history-shattering reform movement".
Sometime during October 31, 1517, the day before the Feast of All Saints, the 33-year-old Martin Luther posted theses on the door of the Castle Church in Wittenberg. The door functioned as a bulletin board for various announcements related to academic and church affairs. The theses were written in Latin and printed on a folio sheet by the printer John Gruenenberg, one of the many entrepreneurs in the new print medium first used in Germany about 1450. Luther was calling for a "disputation on the power and efficacy of indulgences out of love and zeal for truth and the desire to bring it to light." He did so as a faithful monk and priest who had been appointed professor of biblical theology at the University of Wittenberg, a small, virtually unknown institution in a small town.
Some copies of the theses were sent to friends and church officials, but the disputation never took place. Albert of Brandenburg, archbishop of Mainz, sent the theses to some theologians whose judgment moved him to send a copy to Rome and demand action against Luther. By the early months of 1518, the theses had been reprinted in many cities, and Luther's name had become associated with demands for radical change in the church. He had become front-page news.
The article also covers the Issue of Indulgences and the Message of Martin Luther
It is this simple reaffirmation of the ancient Christian "good news," the gospel, that created in the church catholic the reform movement that attracted legions in Germany and other European territories. The movement was propelled by slogans stressing the essentials of Christianity: faith alone (soia fides), grace alone (sola gratia), Christ alone (solus Christus). Many joined because Luther criticized the papacy, which had claimed to have power over every soul. "Why does not the pope whose wealth today is greater than the wealth of the richest Crassus (a wealthy Roman nicknamed "Fats," who died in 53 B.C.) build this one basilica of St. Peter with his own money rather than with the money of poor believers?" (Thesis 87).