From the NET Bible:
who though he existed in the form [morphē] of God did not regard equality with God as something to be grasped
Trinitarian scholar A. T. Robertson (Word Pictures of the New Testament, reprint, Holman Reference, 1958) wants us to believe that morphē means "the essential attributes as shown in the form". It's a typical Trinitarian interpretation, repeated by the NET translators in their footnote ("The Greek term translated form indicates a correspondence with reality. Thus the meaning of this phrase is that Christ was truly God"). Other Trinitarian commentators have taken the same view (e.g. Daniel L. Akin, Jack Cottrell, M. R. Vincent, Glenn Miller, B. D. Smith, J. B. Coffman, James White).
This use of morphē comes from Aristotelian metaphysics. Aristotle used two words for "form" — eidos ("visual image") and morphē ("shape.") In simple terms (for example, a piece of bronze being made into a statue), the "form" is actually is a visual appearance or shape; more often, however, it is some quality of that object.
To illustrate this point, Aristotle uses the example of an axe. The "form" of the axe is not only its shape; it is also the power of the axe to cut wood. Of course, the axe only has this power by virtue of its shape (and we might add other qualities), so the two are closely related. Aristotle regards them as a unity and calls them both together the "form." However, while the Aristotelian definition of morphē is helpful to the Trinitarian interpretation, it does not reflect the broader usage that Paul is drawing upon in Philippians 2. For a more accurate definition we need to consult some standard authorities.
The Liddell-Scott-James Greek Lexicon defines morphē as "form, shape, fashion, appearance, outward form, kind, sort" (definition and semantic range here) and lists Philippians 2:6 under the secondary definition of "fashion, appearance, outward form."
We find the same word in Mark 16:12 ("After this [Jesus] appeared in a different form [morphē] to two of them while they were on their way to the country") and the only other occurrence is in Philippians 2:7. In all three verses the meaning clearly denotes outwards appearance, not "nature", "substance" or "the essential attributes as shown in the form." Standard lexicons such as BDAG, LSJ, EDNT, TDNT, ANLEX, LEHLXX, Louw/Nida and Spicq, do not support the Trinitarian interpretation of morphē as "nature."
Rodney J. Decker (Professor of NT and Greek, Baptist Bible Seminary, PA) is one Trinitarian scholar who candidly admits that the traditional Trinitarian interpretation of morphē is largely the result of theological bias. In an online article (Philippians 2:5-11, The Kenosis) he says:
Lightfoot is a classic example of those who base the meaning of morfhv [MORFH] on Greek philosophy. He explains that it refers to "the specific character" (129); that "morfhv [MORFH] must apply to the attributes of the Godhead" (132). "In Gk philosophical literature, morfhv [MORFH] acquires a fixed and central place in the thought of Aristotle. For him the term becomes equal to a thing's essence (oujsiva) [OUSIA] or nature (fuvsi") [FUSIS]."
Decker also quotes Robert B. Strimple (Philippians 2:5-11 in Recent Studies: Some Exegetical Conclusions, 1979) who was forced to give up the traditional interpretation after realising its futility:
"For years I tried . . . to maintain the view of Lightfoot that Paul here uses morfhv [MORFH] with the sense it had acquired in Greek philosophy, particularly Aristotelian, and which Murray speaks of as 'existence form . . . the sum of those characterizing qualities that make a thing the precise thing that it is.'
Lightfoot wrote: 'though morfhv [MORFH] is not the same as fuvsi" [FUSIS] or oujsiva [OUSIA], yet the possession of the morfhv [MORFH] involves participation in the oujsiva [OUSIA] also for morfhv [MORFH] implies not the external accidents but the essential attributes.'
But I have had to conclude that there is really very little evidence to support the conclusion that Paul uses morfhv [MORFH] in such a philosophical sense here and that my determination to hold on to that interpretation was really rooted in its attractiveness theologically.
In a footnote, Decker quotes another authority who reached the same conclusion:
Feinberg, likewise, notes that "Frankly, the attractiveness of the Gk philosophical interpretation of morfhv [MORFH] is that it gives the theologian about as strong an affirmation of the deity of Christ as is possible. One must, however, be careful that he does not read his theological convictions into the text when they are not there" ("Kenosis," 29-30).
Thus we can reject any suggestion that morphē refers to "nature" or "essential properties" in Philippians 2. Rather than making statements about ontology, Paul is telling us that Jesus was "in the form of God" in exactly the same way as Adam:
Then God said, "Let us make humankind in our image, after our likeness"
The Hebrew words for "image" and "likeness" in this verse are tselem and demûth, which correspond to the Greek words morphē and eikōn. The latter is used by the LXX in Genesis 1:26; cf. Matthew 22:20-21, where Jesus refers to the eikōn of Caesar on a coin ("...the thought of Phil. 2.5ff. relates primarily to the Genesis story and can be understood only by reference to it. The morphe concept presupposes Gen. 1.26...", Oscar Cullman, The Christology of the New Testament, SCM Press, 1959, p.175). None of these words suggest a reference to nature or "essential attributes". The significance of the parallel with Adam will become clear in my next section.