Athanasius I of Alexandria (c. 296–298 – 2 May 373), also called Athanasius the Great, Athanasius the Confessor, or, among Coptic Christians, Athanasius the Apostolic, was a church father and the 20th pope of Alexandria (as Athanasius I). His intermittent episcopacy spanned 45 years (c. 8 June 328 – 2 May 373), of which over 17 encompassed five exiles, when he was replaced on the order of four different Roman emperors. Athanasius was a Christian theologian, a Church Father, the chief defender of Trinitarianism against Arianism, and a noted Egyptian Christian leader of the fourth century.

Conflict with Arius and Arianism, as well as with successive Roman emperors, shaped Athanasius' career. In 325, at age 27, Athanasius began his leading role against the Arians as a deacon and assistant to Bishop Alexander of Alexandria during the First Council of Nicaea. Roman Emperor Constantine the Great had convened the council in May–August 325 to address the Arian position that the Son of God, Jesus of Nazareth, is of a distinct substance from the Father. Three years after that council, Athanasius succeeded his mentor as pope of Alexandria. In addition to the conflict with the Arians (including powerful and influential Arian churchmen led by Eusebius of Nicomedia), he struggled against the Emperors Constantine, Constantius II, Julian the Apostate and Valens. He was known as Athanasius Contra Mundum (Latin for 'Athanasius Against the World').

Nonetheless, within a few years of his death, Gregory of Nazianzus called him the "Pillar of the Church". His writings were well regarded by subsequent Church fathers in the West and the East, who noted their devotion to the Word-become-man, pastoral concern and interest in monasticism. Athanasius is considered one of the four great Eastern Doctors of the Church in the Catholic Church. In his Easter letter of 367, Athanasius was the first person to list the 27 books of the New Testament canon that are in use today. He is venerated as a saint in the Eastern Orthodox Church, the Catholic Church, the Coptic Orthodox Church, the Anglican Communion, and Lutheranism.