The Protestant Reformation
In the 1400s, some western Christians began to publicly challenge aspects of the church. They spoke against the abuse of authority and corruption in Christian leadership. They called for a return to the gospel and a stripping off of traditions and customs like purgatory, the cult of the saints and relics, and the withholding of the communion wine from non-clergy. They began to translate the Bible - then available only in Latin - into the common languages of the people.
However, these early reformers did not have widespread success, and most were executed for their teachings. Legend has it that when Jan Hus, a Czech reformer whose surname means "goose," was burned at the stake in 1415, he called out:
"Today you roast a goose, but in 100 years, a swan will sing!"
In 1517, a German monk named Martin Luther (who bore little resemblence to a swan) posted 97 complaints against the practice of selling indulgences on a church door. He had experienced a personal conversion to the doctrine of justification by faith alone, and also shared many of the ideas of those early reformers.
Growing German nationalism and the invention of the printing press ensured that Luther would have greater protection than his predecessors and his teachings would be spread quickly. He was excommunicated and barely escaped with his life on more than one occasion, but Luther lived out his life spreading the Reformation, and died a natural death.
His ideas had already spread throughout Germany, and similar reforming movements sprung up in England and Switzerland. Soon much of Europe was embroiled in a civil war, with Protestant nationalists fighting Catholic imperialists for religious and political freedom.