The Protestant Reformation in Europe in the first half of the 16th century seriously challenged the Roman Catholic Church. Before Martin Luther publicized his “95 Theses” attacking church corruption in 1517, virtually all Europeans belonged to the Catholic Church, but just 20 years later much of the continent belonged to the Reformed, or Protestant, church. Luther’s critique of the existing church had become a new denomination in its own right. The Catholic Church was forced to respond, and did so in several different ways during a period known as the Counter-Reformation.