Lets seeThe Greek version of the Old Testament (The Septuagint) that Paul quoted so often, was written by monotheistic Jewish Scholars, not by polytheistic Greeks. They did the translation two or three centuries before Christ, and they wrote it for Jew readers who were also monotheists.
So, thinking in God as One Single Person is the basic foundation upon which Paul preached the Gospel.
I agree.
Paul was not arguing against Trinitarism, because Trinitarism did not exist. Not in his mind, not in the mind of the Christians he wrote to.
Isaiah 63:9–16 (KJV 1900) — 9 In all their affliction he was afflicted, And the angel of his presence saved them: In his love and in his pity he redeemed them; And he bare them, and carried them all the days of old. 10 But they rebelled, and vexed his holy Spirit: Therefore he was turned to be their enemy, and he fought against them. 11 Then he remembered the days of old, Moses, and his people, saying, Where is he that brought them up out of the sea with the shepherd of his flock? Where is he that put his holy Spirit within him? 12 That led them by the right hand of Moses with his glorious arm, Dividing the water before them, to make himself an everlasting name? 13 That led them through the deep, As an horse in the wilderness, that they should not stumble? 14 As a beast goeth down into the valley, The Spirit of the LORD caused him to rest: So didst thou lead thy people, To make thyself a glorious name. 15 Look down from heaven, and behold From the habitation of thy holiness and of thy glory: Where is thy zeal and thy strength, The sounding of thy bowels and of thy mercies toward me? are they restrained? 16 Doubtless thou art our father, Though Abraham be ignorant of us, And Israel acknowledge us not: Thou, O LORD, art our father, our redeemer; Thy name is from everlasting.
We have the messenger (angel) of his presence who is called YHWH and God in Scripture - this is the precarnate Christ - word who was God
We have the Holy Spirit
And we have the Father (YHWH)