When it comes to hotly disputed doctrines such as the Trinity, the two sides typically each have certain Bible texts that they claim in support of their position.
For example, in a debate on the Trinity one might expect the Trinitarians to use texts such as Matt. 28:19, John 1:1 and Heb. 1:10-12 in their argument, while the unitarians might use texts such as Mark 12:29, John 14:28 and 1 Tim. 2:5. Each side would go on the offensive with their own texts, and attempt to defend their position against the challenge posed by the other side's texts. (Of course, one would hope that the arguments were more substantial and systematic than simply quoting 'proof texts').
We could draw an analogy with the so-called "swing states" in U.S. presidential elections. Democratic candidates don't bother campaigning in Texas, and Republican hopefuls steer clear of New York, because they know they can't win those states, despite their importance to the electoral vote tally. Instead, they focus on states that could go either way, such as Ohio. Similarly, Trinitarians know that they can't make their case from Mark 12:29, and unitarians know they can't make theirs from John 1:1, so they try to provide a plausible counter-proposal to their opponents' claims and then re-focus the debate on their own biblical "territory."
I would suggest that 1 Cor. 8:6 is one of the Ohio's of the Bible -- a veritable "swing verse" -- because both Trinitarians and unitarians use it to argue positively for their position. On the unitarian side, for instance, the 1877 Christadelphian Statement of Faith cited this verse as a reason for rejecting the doctrine of the Trinity. Moreover, one of the most comprehensive books written in defense of biblical unitarianism is entitled One God and One Lord, presumably taken from this verse. On the other hand, Trinitarians have also used this text to make positive arguments for their position; indeed, this text has become one of the most important in claiming that Paul's theology was a prototype for Trinitarian theology.
The text reads thus: "4 Therefore concerning the eating of things sacrificed to idols, we know that there is no such thing as an idol in the world, and that there is no God but one. 5 For even if there are so-called gods whether in heaven or on earth, as indeed there are many gods and many lords, 6 yet for us there is but one God, the Father, from whom are all things and we exist for Him; and one Lord, Jesus Christ, by whom are all things, and we exist through Him." (1 Cor. 8:4-6 NASB)
Unitarians point out that v. 6 says, "For us there is one God, the Father, from whom are all things" while mentioning Christ separately. Jesus is explicitly excluded from the "one God" and therefore cannot be a person in the Godhead. Jesus is given a subordinate, intermediary role, while the Father is the source of all things. New Testament scholars such as James D.G. Dunn see Ps. 110:1 (a pivotal Christological text for the early church) as the main background to the use of the word "Lord" here. Thus the title emphasizes how the Father has exalted the human Jesus to a heavenly position.
Trinitarians, on the other hand, claim that the text includes the Father and Christ together on the heavenly side of the Creator-creature divide that was all-important in Jewish monotheism. Scholars such as Tom Wright and Richard Bauckham see Deut. 6:4, the Shema, as main background to the language Paul uses here. Indeed, they argue that Paul has rearranged the words of the Shema, "the Lord our God is one Lord," to include Jesus within the identity of the one God.
So which interpretation of this pivotal text is more convincing? For me it is undoubtedly the latter. Firstly, if 1 Cor. 8:6 excludes Jesus from being God, it also excludes the Father from being Lord. But Paul is not using these titles for the Father and Son to the exclusion of one another, but to the exclusion of all other claimants to these titles. What should strike us in 1 Cor. 8:6 is not that Jesus is excluded from being God, but that Paul included Jesus in his argument that "there is no God but one." Surely a mere creature has no place in such discourse!
Secondly, 1 Cor. 8:5 appears to be an allusion to Deut. 10:17: "the LORD your God is the God of gods and the Lord of lords." This is the only place in the Old Testament where "gods" and "lords" occur in the same sentence (Ciampa and Rosner in Commentary on the New Testament Use of the Old Testament, p. 718). Thus it is reasonable that Paul expected his readers to interpret v. 6 with reference to Deuteronomy as well.
Thirdly, as I have argued elsewhere, the use of the Greek preposition dia followed by a genitive requires that we see Christ as participating in creation: "through whom are all things and through whom we exist." The very same language is used of God in Rom. 11:36 (by Paul) and Heb. 2:10. Paul is writing on the nature and existence of gods; there is no indication in the immediate context that he is limiting the scope of his statements to the new creation. Thus, the "all things" should be taken as absolute, or qualified only with "in heaven or on earth" (v. 5).
It is not wrong to say that the man Jesus was exalted to the status of Lord by God (Acts 2:36). However, for early Christians, this exaltation was an impetus for questions about Christ's identity rather than the answer to those questions. 1 Cor. 8:6 shows that for Paul, Jesus' Lordship ties into the very definition of Christian monotheism.