The Treaty of Augsburg in 1555 was significant as it allowed for the coexistence of Catholicism and Lutheranism within the Holy Roman Empire. It established the principle of "cuius regio, eius religio," meaning that the ruler of a territory had the authority to determine its religion. This treaty sought to bring peace after years of religious conflict, notably during the Reformation. It marked a crucial step in recognizing the rights of Protestant princes while also legitimizing Catholic practices, ultimately laying the groundwork for future religious tolerance in Europe.