The Nicene Creed (325/381) emphatically states that the “one God” is the Father. This conforms to the New Testament grammar concerning references to the “one God”. Many Eastern Orthodox theologians follow this ancient tradition by affirming that the “one God” is the Father and not the Trinity. Note the following from Fr. Thomas Hopko:
…in the Bible, in the creeds, and in the Liturgy, it’s very important, really critically important, to note and to affirm and to remember that the one God in whom we believe, strictly speaking, is not the Holy Trinity. The one God is God the Father. In the Bible, the one God is the Father of Jesus Christ. He is God who sends his only-begotten Son into the world, and Jesus Christ is the Son of God.
(From the transcript of the podcast, “The Holy Trinity” - ancientfaith.com/podcasts/hopko/the_holy_trinity#2140.)
Now, to my knowledge, the first theologian to state that the “one God” is the Trinity is Augustine. Fr. John Behr has recently penned some interesting insights on this “new” theological development from Augustine:
Augustine, on the other hand, does not seem to be aware that he is using the term “God” of the Trinity in a radically new manner, one that is not only different but also problematic. The concern of the Cappoadocians, following Athanasius, Origen, and Irenaeus, was not the implications of how one affirms that each divine person is God and the one God, singularly and collectively, but the reverse: how to affirm the one God is Father.
(“Calling upon God as Father: Augustine and the legacy of Nicaea”, in Orthodox Readings of Augustine, p. 161 - see articulifidei.blogspot.com/2016/04/the-employment-of-term-god-some-cogent.html, for further selections from this informative essay.)
I wonder what prompted Augustine to impose “a radically new manner” into the theological reflections on God. It sure seems to me that Eastern Orthodox theologians been more accurate/faithful to the grammar of the New Testament and early Ecumenical Creeds on this issue.
Grace and peace,