Which Pope played a pivotal role in the proceedings of the Council of Chalcedon?

The Council of Chalcedon, which took place in 451 AD, was crucial in defining Christological doctrine, particularly the nature of Christ. Pope Leo I played a significant role in this council through his theological contributions and the authority he wielded as Bishop of Rome.

Pope Leo I is especially noted for the "Tome of Leo," a pivotal document that articulated the dual nature of Christ as both fully divine and fully human. This letter was instrumental in guiding the debates of the council and ultimately influenced the decisions made about Christ's nature, leading to the formulation of the Chalcedonian Definition. His position helped affirm the authority of the papacy in theological matters, emphasizing the role of the Bishop of Rome in resolving doctrinal disputes.

The other popes mentioned did not have a direct impact on the Council of Chalcedon or were not contemporaneous to it. Pope Gregory I, for example, reigned much later in the 6th century, while Pope Leo III and Pope John Paul II have no connections to the 5th-century council. Therefore, it is clear that Pope Leo I's contributions were central to the council's outcomes.