Final answer:
Martin Luther, a German monk, initiated the Protestant Reformation by contesting Catholic Church practices such as the sale of indulgences. He emphasized salvation by faith alone and the Bible as the sole authority, leading to significant changes in Christian doctrine and European society.
Step-by-step explanation:
Martin Luther was a pivotal figure in the history of Christianity, sparking the Protestant Reformation as a response to corruption within the Catholic Church. He was a German monk and professor who famously nailed his 95 Theses to the door of Wittenberg's Castle Church in 1517. These theses critiqued numerous Church practices, particularly the sale of indulgences meant to fund the construction of St. Peter's Basilica in Rome - a practice he found theologically unsound.
Luther's reforms proposed that faith alone was sufficient for salvation, contrasting with the Church's stance on faith and good works. He insisted that the Bible should be the sole source of Christian authority, leading to the notion of 'Sola Scriptura.' His teachings quickly spread throughout Europe, aided by the invention of the printing press, challenging the Catholic Church's doctrines and leading to a fundamental split in Western Christianity.
Pope Leo X, a member of the influential Medici family, was the pontiff who authorized the indulgences that Luther contested. Luther's actions and ideas not only redefined the religious landscape but had profound socio-political implications for Europe. His marriage to Katharina von Bora signified a break from traditional Catholic clerical celibacy, illustrating the extent to which his reforms impacted Christian life and practices.