Douglas Campbell’s Deliverance of God has generated lots of discussion, especially on Romans 1:18-32. The γαρ in 1:18 has been a problem for interpreters long before Campbell came to it. But Campbell’s work is making folks take another look at the particle in this verse.
Koine “traditionalists” (is there a better word?) assert that γαρ is a discourse connector which logically joins two parts of a discourse, normally in an explanatory way. This sense is typically translated “therefore”. Example: I have a broken leg, therefore I will not be playing football. If one only reads the NT, then clearly this is the most frequent usage.
But there is other Greek literature out there. Consider Euripides’ Bacchae. In places like lines 477, 483, and 612, γαρ is used to signal a switch in speaker (like from Dionysus to Pentheus or the Chorus leader to Dionysus). This is evidence for how the particle could function in rhetoric, particularly in a Socratic dialogue. To be fair, just because Euripides used γαρ this way sometimes does not automatically mean that’s what Paul did in Romans 1:18. However, it is evidence that I don’t see many people consider before they dismiss it. A better question for the traditionalists might be Why can’t the γαρ in Romans 1:18 indicate a speaker change?
In addition to Euripides, there’s biblical evidence as well. Consider the translation Greek of the LXX. In Job, when he converses with his “friends”, γαρ is twice used in a change of speaker (Job 6:2; 25:2). Also, by my count there are over 45 instances of γαρ symbolizing a speaker change in LXX Isaiah (tweet me if you want the list and begin discussing who is speaking where in Isaiah). (Maybe this requires an intro to the various voices in Isaiah, but…) One of the clearest examples is Cyrus talking to Yahweh in Isa 45:15— συ γαρ ει θεος, και ουκ ηδειμεν, ο θεος του Ισραηλ σωτηρ (You are the God people cannot see. You are the God who saves Israel. ERV)
Long story short: γαρ is a very small form that gets used in lots of contexts. Identifying what the form means from context-to-context should be determined by those contexts, not by a lexicographic straight-jacket.
So does the γαρ in Romans 1:18 signal a switch from Paul’s voice to the Teacher’s voice? I think the evidence suggests so.