What compelled Martin Luther to nail his ninety-five "theses" to the door of a Wittenburg cathedral, thus launching the Protestant Reformation?

Sales of indulgences by the Church

Explanation:

Martin Luther was a Catholic priest in the German province of Saxony who got baffled with the defilement and avarice he saw inside the Church, and started to lose confidence that the Church would lead him to salvation. In his investigation of the Bible, he decided "that it isn't the Church, however his own individual confidence that will ensure his salvation.