The Council of Nicaea in 325 was called to end an argument that was already tearing the Eastern churches apart. Arius, a presbyter of Alexandria, taught that the Son was the first and highest of all creatures, brought into being by the Father, so that there was a time when he was not. His opponents held that this made the object of Christian worship a creature. Both sides could quote scripture, and both did, at length, without moving each other.
The council broke the deadlock by choosing a word that is not in scripture at all: homoousios, of one being with the Father. That decision is the hinge of Christian doctrinal history. It conceded that the biblical vocabulary alone could not settle the question, because each side was reading the same texts, and it accepted that the church might fix the meaning of scripture with a term from outside it. Everything that follows in the conciliar tradition rests on that move having been made.
It cost the council fifty years of unrest. Homoousios was suspect to many bishops who were not Arians, because it had been used before in senses the church had rejected, and because it was new. The generation after Nicaea spent itself arguing over one word, and the creed of 381 that finally settled the matter kept it. The creed also closed with four anathemas, naming and excluding the Arian formulas, which no later liturgical creed does.