Roman Catholicism was the sole established religion in the Holy Roman Empire until the Reformation changed this drastically. In 1517, Martin Luther challenged the Catholic Church as he saw it as a corruption of Christian faith. Through this, he altered the course of European and world history and established Protestantism. The Thirty Years' War (1618–1648) was one of the most destructive conflicts in European history. The war was fought primarily in what is now Germany, and at various points involved most of the countries of Europe. The war was fought largely as a religious conflict between Protestants and Catholics in the Holy Roman Empire.
Who challenged the Catholic Church in the 15th century? (If the question is unanswerable, say "unanswerable")
unanswerable