On October 31, 1517, Martin Luther, at the time a Roman Catholic monk and theology professor, nailed a protest letter to the Castle Church door in Wittenberg, Germany, where he taught at the local university. The letter is known as his 95 Theses, a protest against the sale of indulgences, among other things. Essentially, indulgences could be bought from the church in order to earn spiritual credits and help get a loved one out of a half-way place that the Roman Catholic Church called Purgatory (a concept that Protestants wholesale reject as fundamentally unbiblical).
One of the driving purposes behind the sale of indulgences was to generate funds with which to build St. Peter’s Basilica in Rome for the Pope. Luther, who had recently come alive to the wonder of the gospel through a personal re-discovery of grace, saw indulgences not only as a direct assault on the gospel, but also as evidence of spiritual corruption in the leadership of the church.
Luther’s theses originally were intended to merely reform the church. However, his protest created a firestorm in the church. He eventually was excommunicated by the Pope, barely avoided assassination, and the Protestant Reformation (from which “protestant” denominations find their roots) was on.