Martin Luther's protest against the sale of indulgences in Wittenberg, Germany, changed the course of European history. The story of how a simple monk came to cause such an uproar among the powerful of 16th century Europe is a fascinating one. It's quite an experience to visit the places where these events occurred.

Wittenberg. See Luther's house, the church doors where the 95 theses were (allegedly) nailed, the oak tree where he burned the papal bull, the churches he preached in, Luther's grave, and related museums.
Read the 95 Theses. When he nailed his 95 theses to the church door in Wittenberg in 1517, he set off shock waves across Christendom for daring to challenge the authority of the pope. (See summary of the 95 Theses.)