What action is Martin Luther best known for in 1517?

Martin Luther is best known for writing the 95 Theses in 1517, which he reportedly nailed to the door of the Wittenberg Castle Church. This document outlined his grievances against the Catholic Church, particularly criticizing the sale of indulgences and other practices he viewed as corrupt. The act of presenting these theses is often considered the catalyst for the Protestant Reformation, as it sparked widespread debate and led to the formation of new Christian denominations out of the desire for reform within the Church. Luther's 95 Theses articulated a call for a return to Biblical teachings and emphasized salvation by faith alone, which resonated with many and significantly impacted religious landscapes in Europe.

The other actions mentioned are associated with different historical figures or events. The Act of Supremacy was established by Henry VIII, the Society of Jesus was founded by Ignatius of Loyola, and the Counter Reformation was a response initiated by the Catholic Church to address the challenges posed by the Protestant Reformation, of which Martin Luther was a significant figure.