Which church figure is known for opposing Pelagianism?

Augustine of Hippo is known for opposing Pelagianism, which is a theological theory that emphasizes human free will and denies the necessity of divine grace for salvation. Augustine argued that humanity is fundamentally flawed due to original sin and that divine grace is essential for individuals to achieve salvation. His work countered the ideas put forth by Pelagius, who taught that humans could choose good without the need for God's grace. Augustine's teachings and writings, especially in works like "On the Grace of Christ" and "On the Nature of Grace," firmly established the doctrine of original sin and the necessity of grace, marking a significant theological shift in early Christianity.

In contrast, Pelagius promoted a different understanding of sin and grace, leading to his association with the very doctrine that Augustine opposed. Gregory the Great and Francis of Assisi, while significant figures in church history, were not primarily known for their opposition to Pelagianism; their contributions lay in other aspects of theology, leadership, and spirituality. Therefore, Augustine's role in opposing Pelagianism is a key milestone in the development of church doctrine on grace and salvation.