On 31 October 1517 a thirty-three-year-old priest in the small German town of Wittenberg wrote a letter that would change the course of history. Addressed to the bishop of his parish, Martin Luther’s letter complained about the Roman Catholic Church’s selling of indulgences, the practice by which the Pope would grant remission from the punishment of sin. The more Luther read the Bible, the more he became convinced that it was not by performing good deeds that one obtained salvation, but by faith alone through God’s grace. It was wrong to claim, he argued in the Ninety-Five Theses contained in the letter, that indulgences could absolve buyers from eternal punishment and grant them salvation.

These indulgences were, of course, a valuable source of income for the Church, especially at a time when Pope Leo X planned to rebuild St Peter’s Basilica in Rome.