Athanasius, the three Cappadocians, and Augustine now elaborate and complete the doctrine of the Trinity [on the basis of the Nicene Confession]. Athanasius understood better than any of his contemporaries that Christianity stands or falls with the confession of the deity of Christ and the Trinity. He devoted his entire life and all his energies to the defense of this truth. He was not fighting for a philosophical problem, but for the Christian religion itself, for the revelation of God, the teaching of the apostles, the faith of the church. The Trinity is the heart and center of Christianity, differentiating it in principle from Judaism, which denies the distinctions within the divine being, and from paganism, which rejects the oneness of God (Ad Serap., I, 28). Athanasius, accordingly, completely avoids the philosophical intermingling of ontology and cosmology. He rejects the gnostic and Arian dualism between God and the world together with its array of intermediate beings (C. Arian., II, 26). The Trinity, he says, is devoid of any admixture of foreign elements; it does not consist in a combination of the Creator with something that has come into being, but is fully and perfectly divine in character (ibid.). The Trinity, therefore, is eternal. In God there are no nonessential features; God does not become anything; he is what he is eternally. As it [the Trinity] always was, so it is and remains; and in it the Father, the Son, and the Spirit (Ad Serap., III 7; C. Arian, I, 18).

 Herman Bavinck, John Bolt, and John Vriend, Reformed Dogmatics: God and Creation, vol. 2 (Grand Rapids, MI: Baker Academic, 2004), 285–286.