proto-orthodox Christology in Earliest NT texts (the Pauline Corpus)

Paul applies the Shema to ''the Father'' and ''Jesus Christ'' in 1 Corinthians 8:6. Paul did not say that the Father and Jesus Christ are the ''one Lord God'' which would conflate the two figures (modalism). What Paul did is to conflate the two numerically distinct figures (Father and Jesus) to the one figure of the Shema (the Lord our God, the Lord is one).

Paul is alluding the Shema in 1 Cor 8:6. Paul applies the 2 different titles in it ( κυριος and θεος) with its adjective εις to 2 different persons (πατρος and Ιησους) in 1 Cor 8:6. In other words, Paul identifies each person as the Lord God. In orthodox Trinitarianism, the Father is God, the Son is God, the Holy Spirit is God, they're not three Gods but one God. Paul's way is proto-orthodoxy. He successfully equated the Father and Jesus with the OT Lord God without conflating their distinct personalities. In 1 Cor 8:6, Paul was able to distinguish the Father and Jesus as 2 persons with the use of different divine titles (Lord/God) indicating the same deity and different prepositions indicating the same function (from him/through him are all things). 2 numerically distinct persons with each of them fully/very God, only differing in words used. John approached orthodoxy already: The Word was "with God" and was "God" at the same time (John 1:1).

The phrases ''heis kyrios'' and ''heis theos'' are absent in the Septuagint. These were coined by Paul himself, deriving them from the Greek version of the Shema. 

First clause of the Shema:

Hear O Israel, The Lord our God 

1 Corinthians 8:6

[one] God [the Father]

In 1 Corinthians 8:6a, Paul used the word heis (one) from the second clause of the Shema. Paul coined the phrase ‘’eis theos’’ (one God) so that the adjective heis (one) modifies theos (God). Paul omits the phrase (Hear O Israel), and the word kyrios (Lord).Then he adds ‘’the Father’’.

Second clause of the Shema: 

The Lord is one 

1 Corinthians 8:6

one Lord [Jesus Christ] 

In Deuteronomy 6:4, the adjective eis (one) is in the predicate case conjoined to the noun kurios (lord) by the copula estin (is). In 1 Corinthians 8:6b, Paul omitted the verb estin (is) and coined the phrase ‘’heis kyrios’’ and adds ''Jesus Christ''.

Paul applies the Shema's 2 divine titles (''Lord'' and ''God'') with its adjective ''one'' to 2 persons (''the Father'' and ''Jesus Christ'') in 1 Corinthians 8:6. Paul did this to equate both the Father and Jesus Christ with OT Lord God. 

Paul used two different divine titles (''Lord'' and ''God'') found in the Shema and applied each divine title to two different figures: the Father and Jesus Christ. Paul applies the divine title ''God'' to ''the Father'' and the divine title ''Lord'' to ''Jesus Christ''. This shows that Paul believed that the Father and Jesus Christ were the same Lord God spoken of in the Shema while also believing that the Father and Jesus Christ were two numerically distinct figures (which later generations of Christians would call ''persons''). This is proto-orthodoxy.

Paul consistently applies to Jesus the divine title ''Lord'' and to the Father the divine title ''God'' in his epistles. This practise stems from the application of the Shema to the Father and Jesus Christ in 1 Corinthians 8:6.

• “Grace and peace to you from God our Father and the Lord Jesus the Messiah.”  Rom. 1:7; 1 Cor. 1:3; 2 Cor. 1:2; Gal. 1:3; Eph. 1:2; Phil. 1:2; Col. 1:2b; 1 Thess.1c; 2 Thess. 1:2; Philemon 1:3
• “Grace, mercy and peace from God the Father and Messiah Jesus our Lord.” 1 Tim.1:2b; 2 Tim.1:2b

Paul held onto a belief that can be characterised as "Semi-modalism" (not modalism per se) or "proto-Orthodox" (proto-Trinitarianism) when later concepts were used to relate to (not to interpret) his christological/theological beliefs.

Another historical explanation is the concept of "Syncretism" wherein two deities "merged" to become a single god. This is common concept in the Ancient Near East (ANE). For instance, the Egyptian god Amun-Ra is the syncretism of the god Amun and the god Ra.

In ancient Israelite beliefs, Jewish scholar and Semitic language professor Benjamin Sommers spoke of the "divine fluidity" of Yahweh wherein he can "fragment" his one self into many bodies on earth so that God was really present in the rock (betyl), or that he really appeared to people and was called an angel in the OT etc. these small-scale incarnations of Yahweh in the OT were distinct from Yahweh who's in heaven. But they're all the same Yahweh.

Then we have the Two Powers theology (Alan Segal, James Mcgrath et al.) in the first century A.D. which held that Two Yahwehs (one in heaven and one visiblel in the earth) coexisted. This was not considered a heresy in Judaism until the latter part of the second century A.D.