St. Ambrose (c. 338 - April 4, 397) was the bishop of Milan who became one of the most influential ecclesiastical figures of the fourth century. He promoted the rights of the church in relation to the imperial state and is counted as one of the four original Doctors of the Church. He was also the teacher of Saint Augustine.

St. Augustine (November 13, 354 - August 28, 430), the bishop of Hippo, was both a philosopher and theologian, as well as an influential church leader in north Africa. He framed the concept of original sin and related teachings on divine grace, free will, and predestination, as well as the theory of the just war. His works remain among the most influential in Christian history.

St. Athanasius (c. 293 - May 2, 373), also known as St. Athanasius the Great, was a theologian who later became the patriarch (“pope”) of Alexandria, a leader of immense significance in the theological battles of the fourth century. He is best remembered for his role in the conflict with Arianism, although his influence covers a vast array of theological topics.

St. John Chrysostom (c. 347 - c. 407), archbishop of Constantinople, is known for his eloquence in preaching and public speaking, his denunciation of the abuse of authority by both ecclesiastical and political leaders, the Liturgy of St. John Chrysostom, his ascetic sensibilities, and his violent opposition to paganism. He is particularly honored in the Eastern Orthodox Church.