The main reason why the religious conflicts erupted in Europe was the Protestant Reformation, which began as a religious movement in the 1500s and culminated with the European wars of religion.
Those who opposed or protested the doctrine of the Roman Catholic Church and wanted to reform it were called Protestants, and the new religion or religious branch of Christianity that emerged became known as Protestantism. The movement interested a lot of scholars, theologians, and nobles around Europe, who, using the power of the new printing press, easily spread their influence and their ideas to a lot of people and regions.
Theologian Martin Luther wrote the Disputation on the Power and Efficacy of Indulgences, also known as the "Ninety-Five Theses," a list in which he questions the authority and legitimacy of the Church and proposes ideas how to reform Catholic doctrine. He nailed this list on the doors of the All Saints' Church in Wittenberg, Germany, and it was quickly copied, reprinted, and spread around the country and the European continent.
Influenced by Luther, French theologian John Calvin became a key figure in the Protestant Reformation with his creation of the Christian theology of Calvinism. In England, King Henry VIII rejected the authority of the Pope and the Church of England separated from the Roman Catholic Church.
These new religious ideas sparked numerous conflicts in Europe; more and more people seemed to be interested in Protestantism and rejected the Catholic Church, claiming that it was greedy, immoral, and oppressive. In Germany, radical reformers supported the German Peasants' War (1525), in which the aristocracy killed thousands of peasants. In France, there was the The Massacre of Saint Bartholomew’s Day (1572), in which numerous French Calvinists (Huguenots) were killed for supporting the marriage between the Princess Margaret (Catholic) to Prince Henry of Navarre (Protestant).
The Catholic Church praised the violence, which was their response to the Protestant Reformation—a period known as the Counter-Reformation or Catholic Reformation. This soon resulted with the European wars of religion, which lasted for thirty years. Some sources claim that witch hunting was most popular during the Counter-Reformation.