The Nicene Creed is the baptismal creed of an eastern church enlarged in order to combine theological interpretation with the facts of the historic faith.
It is possible that Leo's letter to Flavian gave the impulse to put it forward because it contained a parallel to words which Leo quoted from the Old Roman Creed, " born of the Holy Ghost and the Virgin Mary," " crucified and buried," which do not occur in the first Nicene Creed.
Kattenbusch supposes that Anatolius, bishop of Constantinople, or his archdeacon Aetius, who read the creed at the 2nd session of the council, took up the idea that through its likeness to the Roman Creed it would be a useful weapon against Eutyches and others who were held to interpret the Nicene Creed in an Apollinarian sense.
So the Nicene Creed is the analysis of the river of the water of life of which the Sermon on the Mount is a description, flowing on from age to age, freely offered to the thirsty souls of men.
He had considerable knowledge of theology, took a prominent part in the theological controversies of the time, and was responsible for the addition of the clause filioque to the Nicene Creed.
