Martin Luther was a German monk, priest, and professor of theology in the early 16th century.

He suffered a deep spiritual crisis over the question of salvation, which he resolved with his doctrine of 'justification by faith alone.'

Luther was outraged by the sale of indulgences in Germany by the friar Johann Tetzel, who used slogans like 'As soon as a coin in the coffer rings, a soul from purgatory springs.'

In 1517, Luther wrote his 'Ninety-Five Theses,' a list of academic arguments against the sale and theological basis of indulgences.

According to tradition, he posted the theses on the door of the Castle Church in Wittenberg on October 31, 1517.

The Ninety-Five Theses argued that only God could grant remission for sins and that the Pope had no authority over purgatory.

Initially intended to spark an academic debate, the theses were quickly translated into German and, thanks to the printing press, spread throughout the Holy Roman Empire.

This act is traditionally considered the start of the Protestant Reformation.

Luther's initial goal was to reform the Catholic Church, not to break away from it.

The widespread and popular response to his theses quickly escalated the conflict with the papacy.