Traditional Christology notes that the hypostatic feature of the Father is causality. The Father “causes” both Son and Spirit (this is not subordinationism since we are speaking outside of time, if you will). It is not the divine essence that “causes,” but the person of the Father.
The Arians defined the divine essence with causality. They denied Christ was fully God because Christ didn’t “cause” but was caused. (This is similar to the Eunomians who defined the divine essence with unbegotteness). St Athanasius saw this problem clearly and granted the Arians the hypothetical: if the divine essence is associated with causation, and if the Son were fully God, then Son would have to cause “another.” That “another” would have to cause “another,” and so on. The Arians rightly saw that this was polytheism.
As Joseph Farrell notes,
The point of contention for Saint Gregory, as for Saint Athanasios, was the structural subordination imposed upon the divine hypostases by a definition inherently pagan. And notably, a particularly intense controversy centers upon the presence of the logical priority of a category of energies or attributes to any of the divine persons. Such a position was always perceived as Arianism. The resemblance that this energy in the structure of Eunomios bears to the attributes in Augustinian triadology is more than coincidental. Having assumed the definition of simplicity, both Eunomios and Saint Augustine were bound by that definition to produce similar theological structures, even if they were entirely at cross purposes in so doing.
The goal of St Athanasius, then, was to deny that causality belongs to the divine essence. He was to maintain a distinction between essence, will, and person.