Athanasius of Alexandria (c. 293-373) is honored today on Western and Eastern church calendars. He was the 20th bishop of Alexandria, from 328 till 373, but nearly half of his episcopate was spent in exile because, as a defender of trinitarian theology, he was opposed by different Roman emperors who were sympathetic to Arianism. Athanasius had a major role in the first Council of Nicaea which affirmed the "consubstantiality" of the Son with the Father, as opposed to the theology of Arius that affirmed that the Son is subordinate to the Father. "Athenians contra mundum" (Athanasius against the world) is an expression that reflects his courageous integrity in affirming what became orthodox Christian teaching about the nature of God .
The Orthodox Saints website (http://www.abbamoses.com/months/january.html) has this:
"...Even as a child, his piety and devotion to the Faith were so notable that Alexander, the Patriarch of the city, took Athanasius under his protection. As a student, he acquired a thorough education, but was more interested in the things of God than in secular learning, and withdrew for a time into the desert to sit at the feet of Saint Anthony (January 17), whose disciple he became and whose biography he later wrote. On returning to Alexandria, he was ordained to the diaconate and began his public labors for the Church. He wrote his treatise On the Incarnation, when he was only twenty. (It contains a phrase, still often quoted today, that express in a few words some of the depths of the Mystery of the Incarnation: God became man that man might become god.)
"Just at this time Arius, a priest in Alexandria, was promoting his enticing view that the Son and Word of God is not of one essence with the Father, but a divine creation of the Father. This view, which (as Athanasius realized) strikes at the very possibility of mankind's salvation, gained wide acceptance and seemed for a time to threaten the Christian Faith itself. In 325, the Emperor Constantine the Great convoked a Council of the Church at Nicaea to settle the turmoil that had the Arian teaching had spread through the Church. Athanasius attended the Council, and defended the Orthodox view so powerfully that he won the admiration of the Orthodox and the undying enmity of the Arians. From that time forth his life was founded on the defense of the true consubstantiality (homoousia) of the Son with the Father.
"In 326, not long before his death, Patriarch Alexander appointed Athanasius to be his successor, and Athanasius was duly elevated to the patriarchal throne. ... Though the Arian heresy had apparently been condemned once and for all at Nicea, Arius had many powerful allies throughout the Empire, even in the Imperial court, and Athanasius was soon subjected to many kinds of persecution, some local, some coming from the Imperial throne itself. Though he was Patriarch of Alexandria for more than forty years, a large amount of that time was spent in hiding from powerful enemies who threatened him with imprisonment or death. Twice he fled to Rome for protection by the Pope, who in the early centuries of the Church was a consistent champion of Orthodoxy against its various enemies. From his various hiding places, Athanasius issued tracts, treatises and epistles which helped to rally the faithful throughout Christendom to the Orthodox cause... He reposed in peace in 373, having given his entire adult life, at great suffering, to the defense of the Faith of Christ."