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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 Christian theologian and the 20th patriarch 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 Church Father, the chief proponent of Nicene theology against the anti-Nicenes, and a noted Egyptian Christian leader of the fourth century.

Conflict with the Eusebians, as well as with successive Roman emperors, shaped Athanasius's career. In 325, at age 27, Athanasius began his leading role against the Arians as a deacon and assistant to Bishop Alexander of Alexandria. He did not yet play an important role at the First Council of Nicaea. Roman Emperor Constantine the Great had convened the council in May–August 325 to heal the division in the church caused by the dispute between Alexander and Arius. Three years after that council, Athanasius succeeded his mentor as Patriarch of Alexandria. In addition to the conflict with the Eusebians (including powerful and influential 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'). At the Council of Tyre in 335, he was condemned and exiled for violence against the Melitians in his see.

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. Some argue that, 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. Others argue that Origen of Alexandria was the first to list the twenty-seven books of the New Testament in his Homilies on Joshua (only there is a textual variant as to whether or not he included Revelation). Athanasius is venerated as a saint in the Eastern Orthodox Church, the Catholic Church, the Coptic Orthodox Church, the Anglican Communion, and Lutheranism.