Good question and of interest to me as well. I found at least two things that Athanasius reportedly said during the council:
- Said rhetorically to Arius:
How many fathers can you cite for your phrases?
- Said in general?
Jesus that I know as my Redeemer cannot be less than God.
Here's an interesting little paragraph on this that I found here: https://tonyrezk.blogspot.com/2013/11/contra-mundum-i-was-hanging-outwith.html
Accompanying the Pope of Alexandria was a young deacon by the name of Athanasius, who was around the age of 27 at the time of the council. It was he who would emerge as the star of the council as he challenged Arius and proved that his arguments were meager at best. Imagine that, a priest being challenged by a young deacon. Arius would pull bible verses to prove his claims that the son was not equal to the father but Athanasius would respond critically: "How many fathers can you cite for your phrases?” Essentially telling him that the church never believed that Christ was a created being. Furthermore, he would prove to him from the bible that Christ was co-substantial with the father. Moreover, Athanasius was quoted as saying at the council: "Jesus that I know as my Redeemer cannot be less than God". The Issue of Arius’ teachings, as Athanasius saw it, and as his predecessor and teacher Alexander did too, was of salvation. If Christ was a created being then how, as Christians, were we saved. Furthermore, as in one letter that Alexander sent to Arius, “It is necessary that the Father is always the Father. But he is Father of the always present Son, on account of who he is called the Father”. Additionally, what were we to make of John 1:1, in which the apostle John clearly states that Christ is of the same essence of God and that he was not created: “In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God”.
I suppose it's hard to know with accuracy what a young deacon said at a council that took place 1700 years ago... but this is a start. It's also something I plan to keep investigating!