Athanasius of Alexandria died on May 2 in the year 373 and his feast day is celebrated on May 2 in the Catholic and Coptic churches (his feast day in the Orthodox calendar is January 18). Here is a modern icon of Athanasius from St. Athanasius Greek Orthodox Chapel in Alabama:
Athanasius is best known for his opposition to the ideas of Arius.
Arius asserted that God was single and absolute, existing as one being, one person, for all of time. Jesus Christ, the Son of God, was not himself God. Christ was a created being, like God, but not equal to God.
Athanasius was bitterly opposed to the doctrine of Arius, and taught instead that Jesus Christ was an incarnation of God. Athanasius's writings helped to establish the doctrine of the Trinity of Father, Son, and Holy Spirit, who are three persons, but one in substance (the doctrine of "homoousia").
Eventually this belief would defeat Arianism, but Arius was profoundly influential during the fourth century and won many followers.
In his steadfast rejection of Arian's teachings, Athanasius became the subject of a famous saying: Athanasius contra mundum, "Athanasius against the world."
Finally, the views of Athanasius and his allies prevailed, and are enshrined in the Nicene Creed. Here is what the Nicene Creed says about the relationship of God and his Son: "We believe in one Lord, Jesus Christ, the only Son of God, eternally begotten of the Father, God from God, Light from Light, true God from true God, begotten, not made, of one Being with the Father."
The Nicene Creed is used in many Christian churches, but the more simple Apostles' Creed is also widely used. Here is what it says about the begetting of Jesus Christ: "We believe... in Jesus Christ, his only begotten Son, our Lord, who was conceived by the Holy Spirit."
There is also a creed called the Athanasian Creed. Although it bears his name, this creed was not written by Athanasius, yet it does express many of the beliefs he fought for. This creed is not used widely in any church liturgy. Here is just a part of what it says about the relationship among the persons of Trinity: "The Father is uncreated, the Son uncreated, and the Holy Spirit uncreated; The Father is eternal, the Son eternal, and the Holy Spirit eternal. And yet they are not three who are eternal, but there is one who is eternal, just as they are not three who are uncreated, nor three who are infinite, but there is one who is uncreated and one who is infinite."
Both for Christians within the faith, and for people who look at Christianity from the outside, the Trinity is a very difficult doctrine to understand. To explore it further, you might check out a very basic and helpful page at the BBC's Religion and Ethics website.
One group of Christians who reject the doctrine of the Trinity are the Unitarians. For more information about the Unitarians, you can visit the Unitarian Universalist Association of Congregations website. Unitarianism has played an important role in the religious history of America and as you can discover on this page there are some very famous Americans who were Unitarians or Unitarian sympathizers, including one of my own personal heroes, the educational reformer and political philosopher John Dewey.
Somehow, I don't think Athanasius and John Dewey would have gotten along very well...