The Nicene and Dedication Councils were attended by more or less the same people and were only 16 years apart (325 vs 341) but resulted in opposing creeds. The Nicene Creed is pro-Sabellian but the Dedication Creed is anti-Sabellian. What made the difference?
More or less the same people
The Dedication Council was a Council of the Eastern Church and the Nicene Council was almost exclusively Eastern:
At Nicaea, the delegates were “drawn almost entirely from the eastern half of the empire” (LA, 19).
“Very few Western bishops took the trouble to attend the Council (of Nicaea). The Eastern Church was always the pioneer and leader in theological movements in the early Church. It is well known that Hilary, for instance, never really understood the Arian Controversy till he reached the East as a result of being exiled. The Westerners at the Council represented a tiny minority.” (RH, 170)
The Nicene Council “was overwhelmingly Eastern, and only represented the Western Church in a meagre way.” (RH, 156)
The Nicene Creed is pro-Sabellian.
“If we are to take the creed N at its face value, the theology of Eustathius and Marcellus was the theology which triumphed at Nicaea. That creed admits the possibility of only one ousia and one hypostasis. This was the hallmark of the theology of these two men.” (RH, 235) [Eustathius and Marcellus were the two main Sabellians who attended in Nicene Council.]
“The Creed of Nicaea of 325 … ultimately confounded the confusion because its use of the words ousia and hypostasis was so ambiguous as to suggest that the Fathers of Nicaea had fallen into Sabellianism, a view recognized as a heresy even at that period.” (Hanson’s Lecture)
“In the controversies which erupted over Eustathius of Antioch and Marcellus after Nicaea, both thought their theologies faithful to Nicaea—and they had good grounds for so assuming. Both were influential at the council, and Nicaea’s lapidary formulations were never intended to rule out their theological idiosyncrasies.” (LA, 99)
After Nicaea, the Creed was associated “with the theology of Marcellus of Ancyra. … The language of that creed seemed to offer no prophylactic (prevention) against Marcellan doctrine, and increasingly came to be seen as implying such doctrine.” (LA, 96, 97)
“To many the creed seemed strongly to favour the unitarian tendency among these existing trajectories.” (LA, 431) [Ayres uses the term “unitarian” to refer to Sabellianism. For example: “A great deal of controversy was caused in the years after the council by some supporters of Nicaea whose theology had strongly unitarian tendencies. Chief among these was Marcellus of Ancyra.” (LA, 431))
“Simonetti estimates the Nicene Council as a temporary alliance for the defeat of Arianism between the tradition of Alexandria led by Alexander and ‘Asiatic’ circles (i.e. Eustathius, Marcellus) whose thought was at the opposite pole to that of Arius. … Alexander … accepted virtual Sabellianism in order to ensure the defeat of Arianism. … The ‘Asiatics’ … were able to include in N a hint of opposition to the three hypostases theory.” (RH, 171)
It is not “an openly Sabellian creed.” “It is going too far to say that N is a clearly Sabellian document. … It is exceeding the evidence to represent the Council as a total victory for the anti-Origenist opponents of the doctrine of three hypostases. It was more like a drawn battle.” (RH, 172) Ayres says that his conclusions are close to Hanson’s in this regard (LA, 92).
The Dedication Creed of 431 “represents the nearest approach we can make to discovering the views of the ordinary educated Eastern bishop who was no admirer of the extreme views of Arius but who had been shocked and disturbed by the apparent Sabellianism of Nicaea.” (RH, 290)
The Dedication Creed is anti-Seballian.
While Sabellianism asserts only one single hypostasis, meaning one single rational capacity or mind, the Dedication Creed explicitly asserts that the trinity is “three in hypostasis but one in agreement (συμφωνία)” (LA, 118). “One in agreement” indicates the existence of three distinct ‘Minds’.
The Dedication Creed’s “chief bête noire [the thing that it particularly dislikes] is Sabellianism, the denial of a distinction between the three within the Godhead.” (RH, 287)
The Dedication creed is “strongly anti-Sabellian.” (RH, 287)
“The creed has a clear anti-Sabellian and anti-Marcellan thrust.” (LA, 119)
LA = Ayres, Lewis, Nicaea and its Legacy, An Approach to Fourth-Century Trinitarian Theology, 2004
RH = Hanson RPC, The Search for the Christian Doctrine of God: The Arian Controversy, 318-381. 1988