Athanasius of Alexandria (c. 296–298 – 2 May 373) was the 20th bishop 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 leader of the fourth century.

Conflict with Arius and Arianism as well as successive Roman emperors shaped Athanasius' career. In 325, at the age of 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 archbishop of Alexandria. 

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.

Source:  https://en.wikipedia.org/wiki/Athanasius_of_Alexandria

              St. Athanasius - c. 360 A.D.
Against the Heathen St. Athanasius - c. 360 A.D.
On the Incarnation of the Word 
              St. Athanasius - c. 360 A.D.
Deposition of Arius St. Athanasius - c. 367 A.D.     
De Decretis 
                                            St. Athanasius - c. 360 A.D.
The Athanasian Creed