The Saint who fought against heresy

St. Athanasius, also known as Athanasius the Great and Athanasius the Confessor, was a bishop and doctor of the church. He is called the "Father of Orthodoxy," the "Pillar of the Church" and "Champion of Christ's Divinity." Athanasius became one of the most dedicated opponents of the heresy of Arianism. Much of his life was a testimony to the divinity of Jesus Christ.

Arianism was proposed early in the 4th century by the Alexandrian presbyter Arius and was popular throughout much of the Eastern and Western Roman empires, even after it was denounced as a heresy by the Council of Nicaea (325). Arianism position that Jesus was not truly divine. 

Arius’s teaching reduced the Son to a demigod, reintroduced polytheism (since worship of the Son was not abandoned), and undermined the Christian concept of redemption, since only he who was truly God could be deemed to have reconciled humanity to the Godhead.

In 325 the Council of Nicaea was convened to settle the controversy. The council condemned Arius as a heretic and issued a creed to safeguard “orthodox” Christian belief. The creed states that the Son is homoousion tō Patri (“of one substance with the Father”), thus declaring him to be all that the Father is: he is completely divine.

Over the course of his life, Athanasius was banished five times and spent 17 years of his life in exile for the defense of the doctrine of Christ's divinity. During one period of his life, he enjoyed 10 years of relative peace—reading, writing, and promoting the Christian life along the lines of the monastic ideal to which he was greatly devoted. His dogmatic and historical writings are almost all polemic, directed against every aspect of Arianism.

Among his ascetical writings, his "Life of St. Anthony" achieved astonishing popularity and contributed greatly to the establishment of monastic life throughout the Western Christian world. As he grew up, Athanasius befriended many monks and hermits of the desert, including St. Antony

St. Athanasius is often shown as a bishop arguing with a pagan, a bishop holding an open book or a bishop standing over a defeated heretic. He is a patron saint of theologians, and faithful Orthodox and Roman Catholic Christians and hailed to this day as a great Defender of the Faith.

Note on Arianism: Arianism maintained favour among some groups, most notably some of the Germanic tribes, to the end of the 7th century. The Polish and Transylvanian Socinians of the 16th and 17th centuries propounded Christological arguments that were similar to those of Arius and his followers. In the 18th and 19th centuries, the Unitarians in England and America were unwilling either to reduce Christ to a mere human being or to attribute to him a divine nature identical with that of the Father. The Christology of Jehovah’s Witnesses is also a form of Arianism, for it upholds the unity and supremacy of God the Father.

References
https://www.britannica.com/topic/Arianism
https://www.catholic.org/saints/saint.php?saint_id=336
https://www.franciscanmedia.org/saint-of-the-day/saint-athanasius
https://www.franciscanmedia.org/saint-of-the-day/saint-athanasius