How do Trinitarians explain the Athanasian Creed saying that the "one God" is the Trinity rather than the Father as in the Nicene Creed?

In Scripture and the early Creeds, "one God" is used primarily to refer to God the Father: > 1 Cor 8:6 (NIV): yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live. > > Niceno-Constantinopolitan Creed of 381: We believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible. But the Athanasian Creed differs by using the phrase "one God" to refer to the whole of the godhead or Trinity: > And the catholic faith is this: that we worship one God in Trinity, and Trinity in Unity; ... The Father is God, the Son is God and the Holy Spirit is God. They are not three Gods but one God. What explains this shift from thinking first of the Father as God and then to the Son and Spirit as co-equal with the Father, to describing the Trinity as the first and main way we should understand "one God"?