Do any major orthodox Trinitarian theologians or denominations hold that St. Paul was closer to a Biblical Unitarian than a Trinitarian?

Are there any major orthodox (main branch Protestant, Catholic, or Orthodox) Christian theologians or denominations who hold that St. Paul was closer to a Biblical Unitarian? As Andrew Shanks notes, "If any theologian thought this then who would consider them to be a major orthodox Trinitarian theologian? Surely not Trinitarians."

However, if this question is approached from a broader religious perspective, then it is accurate to say Paul was once a Biblical Unitarian, insofar as that term might be applied at the time. This is a certainty, since Paul stated he was Jewish:

“I am indeed a Jew, born in Tarsus of Cilicia, but brought up in this city at the feet of Gamaliel, taught according to the strictness of our fathers’ law, and was zealous toward God as you all are today. (Acts 22:3 NKJV)

Since Paul was Jewish, he should hold to the belief there is one one God, the Father.

If this is true why would he make any reference, "opaque" or otherwise, to the deity of Jesus? In fact, if this question arose, why would someone who believed there was only one God, the Father, not settle the matter by making an explicit statement of the singular deity in which they believed? The logical answer is Paul's Christian experience convinced him Jewish monotheism failed to accurately understand the nature of God. In other words, "One God the Father" did not correctly describe the God in which he came to believe in.

This is evidenced in the events in Athens as reported in Acts:

¹⁶ Now while Paul waited for them at Athens, his spirit was provoked within him when he saw that the city was given over to idols. ¹⁷ Therefore he reasoned in the synagogue with the Jews and with the Gentile worshipers, and in the marketplace daily with those who happened to be there. ¹⁸ Then certain Epicurean and Stoic philosophers encountered him. And some said, “What does this babbler want to say?” Others said, “He seems to be a proclaimer of foreign gods,” because he preached to them Jesus and the resurrection.
(Acts 17)

Regardless of the exact nature of what the Epicurean and Stoic philosophers understood about Paul's message, it was not one of monotheism. Rather, as the text states, they understood Paul's message about Jesus and the resurrection as one which proclaimed foreign "gods" (plural).

Nor could Paul have proclaimed the God of Israel to them:

¹⁹ And they took him and brought him to the Areopagus, saying, “May we know what this new doctrine is of which you speak? ²⁰ For you are bringing some strange things to our ears. Therefore we want to know what these things mean.” ²¹ For all the Athenians and the foreigners who were there spent their time in nothing else but either to tell or to hear some new thing. ²² Then Paul stood in the midst of the Areopagus and said, “Men of Athens, I perceive that in all things you are very religious; ²³ for as I was passing through and considering the objects of your worship, I even found an altar with this inscription: TO THE UNKNOWN GOD. Therefore, the One whom you worship without knowing, Him I proclaim to you: (Acts 17)

The Athenians knew about the God of Israel, so it would be wrong to understand Paul explained the God of Israel was "THE UNKOWN GOD."

The events in Athens show Paul's message there was incompatible with monotheism as it was understood in Second Temple Judaism.

Do any major orthodox Trinitarian theologians or denominations hold that St. Paul was closer to a Biblical Unitarian than a Trinitarian?

If any theologian thought this then who would consider them to be a major orthodox Trinitarian theologian? Surely not Trinitarians.

You say the passages arguing that Jesus is God are opaque:-

"Counter to these sorts of passages are a few opaque passages where some argue St. Paul is claiming Jesus was God."

You want to restrict the subject to only Paul's beliefs. Let me ask you how opaque do you consider these few passages to be:-

1. The Bible says "In the beginning God created the heavens and the earth" (Gen 1:1).

The Apostle Paul says of our Lord Christ: "All things were created by him" (Col 1:16).

2. Paul himself wrote of God: "For of him, and through him, and to him, are all things, to whom be glory for ever" (Romans 11:36).

But of Christ he writes: "Christ loved the church and gave himself for it, ... that he might present it to himself a glorious church" (Ephesians 5:25, 27).

3. The Psalmist says of God: "Whom have I in heaven but thee? And there is none upon earth I desire besides thee" (Psalm 73:25)

The Apostle Paul says: "For me to live is Christ" (Phil 1:21). (What desire then is left for God, if Christ is not God?)

4. God says in the OT: "My son, give me your heart" (Prov 23:26)

But the Apostle Paul says: "I have espoused you to one husband, that I may present you as a chaste virgin to Christ" (2 Cor 12:2). (Again, what love is left for God, if Christ is not God?)

5. God says in the OT: "Cursed be the man that trusts in man, and makes flesh his arm, and whose heart departs from the LORD" (Jeremiah 17:5).

The Apostle Paul writes: "I know whom I have believed, and am persuaded that he is able to keep that which I have entrusted unto him against that day" (2 Tim 1:12).

6. Paul himself says: "God only wise" (Romans 16:27) i.e. only God is wise.

But Paul also writes of our Lord Jesus: "In whom are hid all the treasures of wisdom and knowledge" (Colossians 2:3).

I suspect that inadequate views of the altogether "otherness" of God himself - his glory, his holiness, his purity, his power, his eternity, and all his other characteristics in all their holiness - lie at the root of doubts about the divine nature of our Lord Jesus Christ.

As Charles Brown writes:

"When the Scriptures teach that there is one only God, the meaning will be found to be, not anything of this kind, that there is one Being immeasurably greater, wiser, better, than all others, but that there is one Being, besides whom there is in a sense none else at all, a Being of such a sole, unapproachable excellence, and glory in all things, in being and in all perfection, that if you would bring any creature into comparison with him - it is a matter of indifference whether it be an archangel or a worm - He stands quite alone, in respect of both the one and the other alike." (Charles Brown, "The Divine Glory of Christ", 1982, p25; first published 1868).

I believe this is the God of the Bible. And therefore I believe that where the Scriptures anywhere mention God and our Lord Jesus in the same breathe, as it were, then in such passages there is a clear declaration of the deity of our Lord Jesus Christ. I mean in passages, of which the Scriptures are extremely full including the writings of the Apostle Paul, such as:-

"Grace be to you and peace from God our Father and from the Lord Jesus Christ" (Galatians 1:3).

This sort of mentioning of our Lord Jesus Christ in the same breathe as God the Father is really blasphemy, unless it is that Christ himself is truly God, the glorious Second Person of the Triune Godhead, sharing in that divine "otherness" which sets the Godhead entirely alone from every creature, both worm and archangel alike.

And so it is that whereas God has said in the Old Testament "I will not give my glory to another" (Isaiah 42:8) yet in the New Testament, Revelation 5:13, we read:

"Blessing, and honour, and glory, and power, be unto him that sits upon the throne and unto the Lamb for ever and ever." Amen.