The moment
Constantine called the first empire-wide council of bishops to settle the dispute over Arius, who taught that the Son of God had a beginning and was not eternal like the Father. The council declared the Son 'of one substance' with the Father. The creed recited today in Catholic, Orthodox, Anglican and many Protestant churches is the fuller version issued at Constantinople in 381.
People: Constantine, Athanasius, Arius, Alexander of Alexandria