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 shows
2the 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.
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.