This icon depicts the Fathers of the Ecumenical Council of Nicaea, where the Nicene Creed was written. Emperor St. Constantine the Great sits in the center, surrounded by the holy bishops. The heretic Arius is visible at the bottom, cowering after his heresy was condemned by the council.
St. Nicholas famously struck Arius for his blasphemy, and he was ejected from the council. But in a vision seen by many of the council fathers, Christ appeared and restored the saint to his place. This scene is visible in the top right corner.
The Fathers of this Council are commemorated on the Sunday before Pentecost, on May 29, and the first six Ecumenical Councils are all commemorated together on the Sunday between July 13 and 19.