How do those who hold Jesus = God in the Trinitarian sense account for Paul's reticence to state this clearly and consistently?

Was Paul Reticent?
A first-century Gentile believer would be undoubtedly be shocked by a claim Paul was reticent to proclaim the deity of their Lord Jesus Christ.

For example, regardless of how the ambiguity of Romans 9:5 is understood, a first-century Gentile believer would point to what Paul wrote just a few verses later:

And as Isaiah predicted, “If the Lord of hosts had not left us offspring, we would have been like Sodom and become like Gomorrah.”. (Romans 9:29 ESV)
καὶ καθὼς προείρηκεν Ἠσαΐας εἰ μὴ κύριος σαβαὼθ ἐγκατέλιπεν ἡμῖν σπέρμα ὡς Σόδομα ἂν ἐγενήθημεν καὶ ὡς Γόμορρα ἂν ὡμοιώθημεν

English translations deem it necessary to correct Paul's use of Isaiah and translate his κύριος σαβαὼθ, Lord Sabaoth, as Lord of Hosts, because the Hebrew text of Isaiah 1:9 has יְהוָה צְבָאוֹת. However, the first-century Gentile believer would read Isaiah in Greek, not Hebrew:

And if the Lord Sabaoth had not left us offspring, we would have become like Sodom, and been made similar to Gomorrah. (LXX-Isaiah 1:9 NETS)
καὶ εἰ μὴ κύριος σαβαωθ ἐγκατέλιπεν ἡμῖν σπέρμα ὡς Σοδομα ἂν ἐγενήθημεν καὶ ὡς Γομορρα ἂν ὡμοιώθημεν

Clearly Paul pointed the reader to examine the LXX: he quoted it verbatim. Lord Sabaoth is found in Isaiah over 50 times. It's meaning is impossible to misunderstand:

And they cried out one to another and said: “Holy, holy, holy is the Lord Sabaoth, the whole earth is full of his glory.” (LXX-Isaiah 6:3 NETS)
καὶ ἐκέκραγον ἕτερος πρὸς τὸν ἕτερον καὶ ἔλεγον ἅγιος ἅγιος ἅγιος κύριος σαβαωθ πλήρης πᾶσα ἡ γῆ τῆς δόξης αὐτοῦ

Only when Romans is translated from the anachronistic position the first-century Gentiles readers were expected to ignore what Paul actually wrote and understand it from the Hebrew text can Paul's explicit identification of Jesus as Lord Sabaoth be misunderstood as in an English Bible. In actuality, it is impossible to read Isaiah in the Greek translation and not find numerous statements where Paul clearly and explicitly and purposely equates the Lord Jesus Christ as the God, Lord Sabaoth. Moreover, since Sabaoth is a transliteration of צְבָאוֹת, even if the first-century Gentile heard Isaiah read in Hebrew, the Jewish convention of replacing YHVH with adonai, would be heard as Adonai Sabaoth.

To both the first-century Gentile and Jew, Lord Sabaoth and Lord are how God is identified in Isaiah. If that is a "misunderstanding" of the Old Testament, then it is one which Paul purposely uses and makes no attempt to correct a reader from "mistakenly" understanding what is unmistakably explicit: Jesus is the God Isaiah wrote about.

Only when Paul's writings are examined as if the audience understood the Old Testament in Hebrew, something which was historically impossible, can it be said Paul was reticent.

Why the frequency of God as Father?
A second aspect to this issue is made by contrast: the deity of the Father is found in every one of Paul's letters. This is seen as the de facto proof that Paul believed only the Father was God.

This dynamic in Paul's letters should be placed in the context of a first-century church made up mostly of Gentiles who came to believe as adults. In other words, they had been raised and believed in the reality of many gods. Even a casual look at Greek mythology would cause one to call this extreme polytheism,¹ something the Bible specifically mentions:

Now while Paul was waiting for them at Athens, his spirit was provoked within him as he saw that the city was full of idols. (Acts 17:16)

Ancients historians Lucian, Livy, Pliny, Petronius, and Xenophon all made note of the number of idols and religious feasts. We are not talking about a few gods, but a god for any and everything imaginable...except the God of Israel. As church history shows²the problem for the Gentile Christians was the deity of the Father. Paul demanded the Gentiles believe the Father, whom the despised Jews called their God, was also God to and for them. And so each and every time he wrote, he reminded them they also must accept the Father as God.

This was the Rubicon Paul demanded his Gentile recipients cross. The modern position that a first-century Gentile believer would immediately abandon polytheism and accept the Father, the God of the Jewish people as the only God is contrary to Scripture. Furthermore, the position a first-century Gentile would also immediately not believe Jesus Christ and the Holy Spirit were God, is also contrary to what the Bible states:

¹¹ And when the crowds saw what Paul had done, they lifted up their voices, saying in Lycaonian, “The gods have come down to us in the likeness of men!” ¹² Barnabas they called Zeus, and Paul, Hermes, because he was the chief speaker. (Acts 14)

Is it even remotely possible for people who were willing to accept Paul and Barnabas as god, to deny the deity of the Holy Spirit who filled their hearts with the love of God and worked miracles through them? Is it likely these people would believe their Lord Jesus Christ had made atonement for their sins, made them children of God, and was now at the right hand of God making intercession for them, was not Himself God?

The claim first-century Gentile Christians would reject the deity of both the Holy Spirit and the Lord Jesus Christ and instead believe the God the Father of the Jewish people was the only God can only be seen as eisegesis and contrary to history.

<sub>1. Those who deny the "tripleness" of God (that is the meaning of Tertullian's trinitas) point out how trinity is not used in the Bible; yet they maintain the Bible forbids polytheism, a term which is also absent from the Bible. The Bible never uses either label, but it does define what those labels represent. An argument against trinity on the basis of the "missing" label is as spurious as an argument against polytheism based on the absence of the word.
2. The Maricon heresy denied the Hebrew God was the God who sent Jesus.</sub>

Although I would not read "reticence" into Paul's apparent lack of an explicit trinitarian doctrine, I agree that the idea is not one that he clearly addressed. The reason is simple: he did not need to address it because it had not arisen yet in controversy. Paul clearly addressed issues like the circumcision of gentile Christians because this was a major issue in the early church. He addressed Jesus' Christhood because it was indeed essential to his messages. The Trinity did not become controversial until later, at least as far as we know from the writings of the Church Fathers.

Admittedly this explanation is not normally offered by those who "hold Jesus is God in the Trinitarian sense and that this belief is crucial to the Gospel message." After all, if the belief were crucial to the Gospel message, it ought to be more clearly expressed by Paul than it is. However it might well be offered by trinitarian Christians who believe the Holy Spirit worked through the Ecumenical Councils to establish orthodox Christian doctrine and to exclude those who deny the Trinity.

Paul simply didn't have special revelation into the fact that Jesus is God in the Trinitarian sense that needed to be related at the time of the Gospels. And God didn't need to reveal that to the people who St. Paul was writing to at the time.

It is only through sifting and winnowing good doctrine from bad that the Church came up with the firm doctrine of the Trinity, St. Paul certainly didn't vacillate on "testing everything" and "keeping what is good". There are a thousand other doctrines that needed to be defined at Church councils, obviously the Trinity and Tripartite Nature of Christ were first.

The principles which guide the development of all doctrine are the same, they're guided by the Holy Spirit and either accepted by believers in the holy catholic faith as defined by the Nicaean Creed or they're not, which is why this entire line of questioning is erroneously put before all Christians;

The Protestant principle is: The Bible and nothing but the Bible; the Bible, according to them, is the sole theological source; there are no revealed truths save the truths contained in the Bible; according to them the Bible is the sole rule of faith: by it and by it alone should all dogmatic questions be solved; it is the only binding authority. Catholics, on the other hand, hold that there may be, that there is in fact, and that there must of necessity be certain revealed truths apart from those contained in the Bible; they hold furthermore that Jesus Christ has established in fact, and that to adapt the means to the end He should have established, a living organ as much to transmit Scripture and written Revelation as to place revealed truth within reach of everyone always and everywhere

Tradition and Living Magisterium - Catholic Encyclopedia

I could use this same answer for any application of orthodox Trinitarian belief to scripture. The Church believes the Holy Spirit guides her and the doctrine of the Trinity rests on the authority of the teaching office of the Church who get their authority as the the Apostles, which Paul admits he is just not the best representative of.

Last of all, as to one born abnormally, he appeared to me. 9 For I am the least[d] of the apostles, not fit to be called an apostle, because I persecuted the church of God.

1 Cor 15:8-9

So, instead of asking, why did does scripture appear to contradict some theological point. Why not ask, why was scripture written this way? Failing to do this is like trying to drink a shake with a 4 inch straw, you're just never going to get to the bottom of it.

If I understand the implicit Unitarian objection in the Q sufficiently, you seem to refer to how God made Moses = (like) God as Ex 7:1 says:

And the Lord said to Moses, “See, I have made you like God to Pharaoh, and your brother Aaron shall be your prophet.

Yes, Jesus in his human nature IS God's agent like Moses (especially as prophet) as well as human representative like Adam. But according to Trinitarians Jesus is also more than an agent since in his divine nature Jesus = Son of God (The Word) in the ontological sense, who in Advent realized the divine mission by taking on a body like ours 1) in full solidarity with us (struggling and suffering), 2) for our salvation (by his death on the cross), 3) to be the Perfector of humanity (so we can conform to Christ, receive power to fight our sinful nature, and nourished by His body) and 4) for our hope (resurrected glorified body). If Jesus was merely an agent like Moses it would not have been enough to fulfill these 4 purposes and it would greatly lessen the value of God's declaration of His love for us. Only God can fulfill those 4 purposes for us.

Even though Paul was not as explicit as John, Trinitarians believe that the true nature of Jesus was understood and known by Paul in his mind. But since we don't have access to Paul's mind we can only glean from his extant writings. St. Paul received prophetic revelation on who Jesus is because God specially inspired him to teach and to write letters, a subset of which were preserved providentially and later recognized as canonical. His writings are saturated with OT commentaries, consistent with his rabbinical training and in the style understood by his audience who expected practical answers in connection with the "economy of salvation" (i.e. divine mission, the 4 purposes above) instead of Greek philosophy analysis of the exact nature of Jesus.

But later theologians faced a different sort of concern and in response they tried to construct a Christological definition that STILL respected Paul's understanding but at the same time more fully described who Jesus is in terms of "One Who" (eternally begotten Son of God, the Word) and "Two What" (fully human and fully God) to INTEGRATE not only Paul's inspired statements but also all of what Jesus did and said about himself said, as well as what other NT writers wrote about Jesus.

You cannot read this fuller Chalcedonian understanding into our reading of Paul, but we can believe that were Paul to be present at the councils and were Paul to have the benefit of reflecting for a few hundred years, trying to restate in philosophical terms the Jesus whom he knew as prophetically revealed to him in his heart, he would have agreed that the Chalcedon definition BEST captures the answer to the "who" and the "what" of Jesus in order to stave off the dozen or so historical heresies.

An example showing a hint that Paul would have understood Jesus in Chalcedon terms is Romans 8:3 which clearly links Jesus as Son of God (fully God) with an incarnated body like ours (fully human):

The law of Moses was unable to save us because of the weakness of our sinful nature. So God did what the law could not do. He sent his own Son in a body like the bodies we sinners have. And in that body God declared an end to sin’s control over us by giving his Son as a sacrifice for our sins.

If we read the whole Romans carefully, no one can say that St. Paul was reticent but instead was very emphatic in his tone and understanding of Jesus. I think Trinitarians can easily show that every statement that St. Paul made can be reconciled with Chalcedon in a manner that does NOT add to God's revelation (except for clarity). Don't blame Paul for not saying in better clarity who/what Jesus is but maybe we can lament that God did not providentially included Chalcedon in NT so Unitarians don't have to wonder at the orthodoxy of Chalcedon.

For a short explanation of how a Christian theologian can show continuity between Paul and Chalcedon without adding to the 1st century revelation, please listen to minutes 3:17 to 10:30 of the Church Grammar podcast episode Thomas Joseph White on Tough Trinity Questions. Fr. Thomas Joseph White is a Dominican who specializes in the development of the doctrines of Trinity and Christology in the early Church Fathers, and is quite conversant with modern attempts to redefine Trinity and Christology. The book referenced in the episode is The Trinity: On the Nature and Mystery of the One God.

Paul clearly states the Deity of Christ and is not at all reticent to do so.

1. Romans 9:5

Whose are the fathers, and of whom as concerning the flesh Christ came, who is over all, God blessed for ever [KJV]

2. Acts 20:28

... feed the church of God, which he hath purchased with his own blood.[KJV]

3. Ephesians 5:5

... the reign of the Christ and God [YLT]

4. 2 Thessalonians 1:12

... according to the grace of our God and Lord, Jesus Christ.[YLT]

5. 1 Timothy 5:21

... before the God and Lord, Jesus Christ [Literal]

6. Timothy 4:1

... before the God and Lord, Jesus Christ [Literal]

7. Titus 2:13

... the glory of our great God and Saviour, Jesus Christ [YLT]

It is necessary to examine the translation of these passages in detail, particularly in regard to the Greek idiom referred to as the TSKS construction ('the'/substantive/kai/substantive) which was the subject of research carried out by Granville Sharp and is now referred to as 'Sharp's Rule'.

My sources are :-

(1) Granville Sharp on ‘The Definitive Article’ first given in a sermon and edited in 1792.

(2) The Orthodox Churchman’s Review of Sharp’s paper, Feb 1803 No 2 Vol 4 P105

(3) The Englishman’s Greek New Testament, Bagster 1877 containing the Stephen’s Greek text of 1550 and an interlinear, literal translation.

(4) Young’s Literal Translation Third Edition 1898

(5) Daniel B Wallace Beyond the Basics Zondervan 1996 pp 270-290 (B) The article with Multiple Substantives Connected by Kai, (Granville Sharp Rule and Related Constructions)

To summarize, your question is twofold: In the Pauline Corpus...

1. Why wasn't Paul bold (or why was he reticent)?
2. Why wasn't Paul clear about Jesus' Divinity?

Let's look at question 2 first:

Why wasn't Paul clear about Jesus' Divinity?

I'm going to include a long section from my Dogmatics notes here. I admit that there's some here that is outside the Pauline Corpus. But it's inconsistent to omit these sections (especially from the tetraeuangelion) since they are foundational to what Paul speaks about in his own writings:

6. The godhead of the Son is attested

a)  By various witnesses.
    1)  Messianic prophecies.
        cf  Is 9:6; Mic 5:2.
    2)  John Baptist.
        cf  Jn 1:27,30,34.
    3)  Angels.
        cf  Lk 1:35; 2:11,14.
    4)  Demons.
        cf  Mt 8:29; Mk 1:24; 3:11; Lk 4:41.

b)  Particularly by the Father.
    cf  Mt 3:17; 17:5; Jn 5:32,37; 8:18; 12:28; 1 Jn 5:9; He 1:5.

c)  By Jesus himself, claiming
    1)  To be the Father's Son.
        cf  Mt 11:27; Mk 14:61,62; Jn 3:16; 4:25–26; 5:17,18; 9:35–38; 14:9–11; 17:21–23
    2)  Divine attributes.
        cf  Jn 8:59; 17:5; Re 1:8,17,18;—Mt 28:18;—Mt 18:20; 28:20;—Jn 5:26; 10:18; 14:6.
    3)  Divine works.
        cf  Jn 10:37,38; 14:11; 15:24.
        cf  Jn 5:21,22; 6:39,40; 11:25;—jn 8:12; 12:46.
    4)  Divine honor.
        cf  Jn 5:23.

d)  And by the apostles, ascribing to Him
    1)  Divine names (ὀνοματικῶς).
        cf  Mt 14:33; 16:16; Jn 1:1,14,18,49; 6:69; 20:28; Ac 8:37; 20:28; Ro 9:5; Col 2:9; 1 Tim 3:16; Tit 2:13; 1 Jn 5:20.
    2)  Divine attributes (ἰδιωματικῶς).
        cf  Jn 1:1,2; Col 1:17; He 13:8;—Jn 2:25; 21:17;—1 Pe 3:22.
    3)  Divine works (ἐνεργετικῶς).
        cf  Jn 1:3; Col 1:16;—1 Cor 8:6; He 1:3; Re 11:15;—Philip 3:21;—Ac 10:42; Ro 2:16; 2 Cor 5:10.
    4)  Divine honor (λατρευτικῶς).
        cf  Ac 5:31; 7:55,56; Eph 1:20; Philip 2:9–11;—He 1:6; Re 5:11–14.

From these examples (even if, for some strange reason) one stays only within the Pauline corpus, it is not difficult to see that, yes, indeed Paul sees Jesus as true/fully God.

Connected to this fact, is the continued teaching that there is only one God. And thus salvation only comes from this one God revealed in three persons. Again, a selection from my good 'ol dogmatics notes:

5. God is One regarding essence and Number (unitas Dei).

a) On the fact cf Deut 6:4; 4:35; Is 41:4; 43:10–11; 1 Cor 8:4–6; Ga 3:20; Eph 4:4–6.

Isn't it interesting that the passages that speak about the unity of God are all from the Pauline corpus? This shows us that Paul was not at all shy about saying and showing that Jesus was true God.

Why wasn't Paul bold (or why was he reticent)?

The answer to this question is context. When we read Paul in his context we quickly see that his fear is not because he is not convinced of Jesus' Divinity. Instead it's because of his own innate frailty as a human being and because of the abusive treatment he received when he preached the gospel.

As an example of his own frailty, let's look at 2 Corinthians:

“<3> Praise be to the God and Father of our Lord Jesus Christ, the Father of compassion and the God of all comfort, <4> who comforts us in all our troubles, so that we can comfort those in any trouble with the comfort we ourselves receive from God. <5> For just as we share abundantly in the sufferings of Christ, so also our comfort abounds through Christ. <6> If we are distressed, it is for your comfort and salvation; if we are comforted, it is for your comfort, which produces in you patient endurance of the same sufferings we suffer. <7> And our hope for you is firm, because we know that just as you share in our sufferings, so also you share in our comfort. <8> We do not want you to be uninformed, brothers and sisters, about the troubles we experienced in the province of Asia. We were under great pressure, far beyond our ability to endure, so that we despaired of life itself. <9> Indeed, we felt we had received the sentence of death. But this happened that we might not rely on ourselves but on God, who raises the dead.” (2 Cor. 1:3–9 NIV11-GKE)

Notice how Paul speaks. First he mentions that he is "oppressed" (ⲑⲗⲓⲃⲱ—vs. 6). Twice he mentions the "sufferings" he went through (ⲡⲁⲑⲏⲙⲁⲧⲱⲛ). In verse 8, he repeats the "oppression" he spoke of earlier (ⲑⲗⲓⲯⲉⲱⲥ). He then says that they were so weighed down (ⲉⲃⲁⲣⲏⲑⲏⲙⲉⲛ) that they despaired (ⲉⲝⲁⲡⲟⲣⲏⲑⲏⲛⲁⲓ) even of living. Notice, how in all of this, his fear was based both on his own human weakness and the fierce opposition he faced.

But also note the basis of his hope and optimism: just as we are comforted by God (“παρακαλούμεθα αὐτοὶ ὑπὸ τοῦ θεοῦ·” (Κορινθίους β 1·4 THGNT-T)), so also, through Christ, also consolation overflows to us (“οὕτως διὰ τοῦ χριστοῦ περισσεύει καὶ ἡ παράκλησις ἡμῶν” (Κορινθίους β 1·5 THGNT-T)).

Summary: It's important to read any book, and especially the Pauline Corpus in context. Paul was not reticent because of his theological convictions. He was reticent because of his internal frailty and because of external abuse.