During which century did the spread of Protestant ideas commence?

The spread of Protestant ideas really took off in the 16th century, thanks to a key figure: Martin Luther. In 1517, Luther, a German monk, famously nailed his "Ninety-Five Theses" to the door of a church in Wittenberg. These theses challenged some of the practices of the Catholic Church at the time, particularly the sale of indulgences, which were essentially pardons for sins. Luther's ideas were quickly spread throughout Europe, aided by the relatively new invention of the printing press. This allowed for mass production of pamphlets and books, making his arguments accessible to a much wider audience. While there were earlier rumblings of dissent within the Church, it was Luther's actions and the subsequent movement he inspired that truly ignited the Protestant Reformation in the 16th century. The 15th century was a bit too early, and the 17th and 18th centuries saw the further development and diversification of Protestantism, but the initial spark and widespread adoption occurred in the 1500s.