The main impact of the Reformation on the Catholic Church was the pivotal challenge it posed to its authority and practices. Initiated by figures like Martin Luther and John Calvin in the early 16th century, the Reformation led to the emergence of Protestant denominations, which rejected certain Catholic doctrines and ecclesiastical authority. This fracturing of Christianity prompted the Catholic Church to respond with the Counter-Reformation, initiating reforms such as the Council of Trent. The movement also encouraged greater emphasis on personal faith and scriptures, leading to significant theological, political, and cultural shifts in Europe that shaped the modern religious landscape.