The Protestant Reformation was the 16th-century schism within Western Christianity initiated by Martin Luther, John Calvin and other early Protestants. It was sparked by the 1517 posting of Luther's Ninety-Five Theses. The efforts of the self-described "reformers", who objected to ("protested") the doctrines, rituals, leadership and ecclesiastical structure of the Roman Catholic Church, led to the creation of new national Protestant churches. The Reformation was precipitated by earlier events within Europe, such as the Black Death and the Western Schism, which eroded people's faith in the Catholic Church and the Papacy that governed it. This, as well as many other factors, such as spread of Renaissance ideas and inventions, such as the invention of the printing press, and the fall of the Eastern Roman Empire, contributed to the creation of Protestantism.

The Roman Catholic Church responded with a Counter-Reformation initiated by the Council of Trent—the most important ecumenical council since Nicaea II 800 years earlier (at the time, there had not been an ecumenical council since Lateran IV over 300 years earlier, a length only to be matched by the interval between Trent and Vatican I)—and spearheaded by the new order of the Society of Jesus (aka Jesuits) specifically organized to counter the Protestant movement. In general, Northern Europe, with the exception of Ireland and pockets of Britain and the Netherlands, turned Protestant. Southern Europe remained Roman Catholic, while fierce battles which turned into warfare took place in central Europe.