The Council met at Nicea (in northern Turkey, not far from Istanbul) in 325. Although Athanasius probably did not participate officially in the Council, he played a major part behind the scenes. The Council decided against Arius but the debate did not go away and another Council was called in 381 in Constantinople (Istanbul). This Council edited the statement of the Council of Nicea into what we now call the Nicean Creed. [The Athanasian Creed, which is printed in the back of our Prayer Books, was not written by him but emerged later in the western church.]
Meanwhile back in Alexandria Athanasius became bishop in 328. However various political and theological power struggles followed and he was banished and recalled about five times in the next forty years.
His big contribution was to help explain the nature of Christ. He argued for the key word “homoousios” which meant that Christ was of one substance (consubstantial) with the Father. That is, it meant that Christ was fully divine.
His most famous writing is called “On the Incarnation of the Word” in which he argues that it was necessary for the Christ to be fully divine in order to restore the divine image to corrupted humanity, to die humanity’s death on the cross and to raise an immortal and incorruptible humanity to life.
The debates about the nature of Christ continued until the Council of Chalcedon, in 451, agreed on a definition of the two natures of Christ – that he was truly God and truly human. [More on Chalcedon here and here]. It was Athanasius who helped clarify that the Son was “of one being with the Father”. You can read more about Athanasius here.
Dale