Thanks, Paul the Uncertain.
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I read through the two articles that Neil Godfrey provided links for (thanks again, Neil!)
* Cover, Michael Benjamin. “The Death of Tragedy: The Form of God in Euripides’s Bacchae and Paul’s Carmen Christi.” Harvard Theological Review 111, no. 1 (January 2018): 66–89.
https://sci-hub.se/10.1017/S0017816017000396
* Bockmuehl, Markus. “‘The Form of God’ (Phil. 2:6) Variations on a Theme of Jewish Mysticism.”
https://sci-hub.se/10.1093/jts/48.1.1
It was pointed out that Irenaeus also saw Phil 2 in terms of "an Adam Christology" as well. In Book 5, Ch 16 of "Against Heresies":
http://www.earlychristianwritings.com/t ... book5.html
2... For in times long past, it was said that man was created after the image of God, but it was not [actually] shown; for the Word was as yet invisible, after whose image man was created, Wherefore also he did easily lose the similitude. When, however, the Word of God became flesh, He confirmed both these: for He both showed forth the image truly, since He became Himself what was His image; and He re-established the similitude after a sure manner, by assimilating man to the invisible Father through means of the visible Word.
3. And not by the aforesaid things alone has the Lord manifested Himself, but [He has done this] also by means of His passion. For doing away with [the effects of] that disobedience of man which had taken place at the beginning by the occasion of a tree, "He became obedient unto death, even the death of the cross; " rectifying that disobedience which had occurred by reason of a tree, through that obedience which was [wrought out] upon the tree [of the cross]. Now He would not have come to do away, by means of that same [image], the disobedience which had been incurred towards our Maker if He proclaimed another Father. But inasmuch as it was by these things that we disobeyed God, and did not give credit to His word, so was it also by these same that He brought in obedience and consent as respects His Word; by which things He clearly shows forth God Himself, whom indeed we had offended in the first Adam, when he did not perform His commandment. In the second Adam, however, we are reconciled, being made obedient even unto death. For we were debtors to none other but to Him whose commandment we had transgressed at the beginning.
It doesn't answer the question of whether Paul saw pre-existence or not in Phil 2:6. Obviously Irenaeus thought that Christ pre-existed since it was the orthodox position of his time. Irenaeus doesn't reference "form of God", but he does use "image of God" leading up to his reference to Phil 2, for what that's worth.
I wasn't impressed by Bockmuehl's article, which was too much involved with the theological issues. For example, from Page 21:
It is theologically incorrect, therefore, to assume that the form of God is constituted by the earthly appearance of the incarnate Jesus, as though the 'form of God' were somehow directly identical with the 'form of a slave'. This mistake continues to be encountered even among systematic theologians.
Since I'm not interested in the theology of the question, I didn't find much value in that article. But Cover's article on the echoes of Euripides’s Bacchae within Paul's epistles was much more relevant.
Cover sees Paul's use of the expression "form of God" as a wink to a pagan audience familiar with Euripides’s Bacchae. That makes sense to me. We tend to think of religions as monoliths: an adherent believes in THIS god/gods and disbelieves in THAT god/gods. That line was much less strong in Roman times. The Egyptian gods were Roman gods, just rebranded. It would have been easy for them to see Jehovah as a rebranding of Jupiter and Jesus as a rebranding of Dionysus or Pythagoras. If "form of God" was a near-synonym to "image of God", it might have been close enough to pass by the sensibilities of his Jewish audience while also being meaningful to his pagan audience.