Who played a significant role in the proceedings of the Council of Ephesus?

Cyril of Alexandria played a significant role in the proceedings of the Council of Ephesus in 431 AD. He was a staunch defender of the orthodox Christian faith and was a key figure in the Christological debates of the time, particularly against the Nestorian position, which emphasized a separation between the divine and human natures of Christ.

Cyril's leadership at the Council was critical; he presided over many of its sessions and was instrumental in articulating and defending the position that Mary should be recognized as Theotokos, or "God-bearer," affirming the unity of Christ's natures. His theological writings, such as the "Mystagogical Catecheses," provided a foundational framework for the council's discussions.

Additionally, his unwavering opposition to Nestorius helped solidify the decisions made at the Council, leading to the condemnation of Nestorianism and the affirmation of the Council’s key decrees. Cyril’s influence on the proceedings and outcomes of the Council established him as one of the foremost theologians of his time, significantly shaping the development of Christological doctrine in the early Christian church.