In displaying the mind of Christ, Paul begins with one of those sublime sentences whose essential intent and meaning seem clear as can be yet whose parts are full of mystery and wonder. The reason for this is simple enough; on the basis of what was known and came to be believed about Jesus' earthly life, Paul is trying to say something about what could not be observed yet came to be believed about Christ's prior existence as God. What is essential is this: In his prior existence as God, Christ demonstrated what equality with God meant, not by taking advantage of it for himself but by emptying himself, by taking the role of a slave/servant in becoming one of us. All of this, in the present context, is to portray the ultimate expression of "do nothing out of selfish ambition or vain conceit." What that "mind" entails is spelled out in the narrative that follows.
The details that need special attention are four: (1) Why does Paul say "in the form" of God (NIV margin) rather than simply say "as God"? (2) What does the word harpagmos mean (NIV something to be grasped; put above as "taking advantage of")? (3) What did it mean for Christ to "empty himself" (NIV made himself nothing)? (4) Why in human likeness rather than "in his humanity" or something similar? Other details of significance will be noted in the process of looking at each of these matters in turn.
First, the opening phrase, "who, being in the form [morphe] of God," expresses as presupposition what the rest of the sentence assumes: that it was the preexistent Christ who "emptied himself" at one point in our human history. Why then did Paul use morphe, which primarily refers to the "form" or "shape" something takes? The answer lies in what Paul is about in this sentence. His urgency is to say something about Christ's "mindset" in both expressions of his being, first as God and second in his humanity. But in the transition from Christ's "being God" to his "becoming human," Paul expresses by way of metaphor the essential "shape" of that humanity: Christ "took on the `form' of a slave." Since morphe can denote "form" or "shape" in terms of both the external features by which something is recognized and the characteristics and qualities that are essential to it, it was precisely the right word to characterize both the reality (his being God) and the metaphor (his taking on the role of a slave). On the basis of Christ's resurrection and ascension, his earliest followers had come to believe that the One whom they had known as truly human had himself known prior existence in the "form" of God—not meaning that he was "like God but really not" but that he was characterized by what was essential to being God. This understanding (correctly) lies behind the NIV's in very nature God.
Second, in order to highlight the astonishing nature of the incarnation, Paul resorts to a typical "not/but" contrast. Reading such a sentence straight through without the "but" clause always helps to get at what is essential; in this case, "who, being in the form of God, . . . made himself nothing by taking the form of a servant/slave." That is glory indeed; but to heighten the glory Paul emphasizes two realities: first, that "being in the form of God" means being equal with God; second, that in Christ's "being in the form of God/being equal with God" he displayed a mindset precisely the opposite of "selfish ambition" and empty glory (v. 3). To accent this second point he uses an extremely rare (negative) word, harpagmos, which depicts the opposite of "in humility consider others better than yourselves" (v. 3).
Harpagmos is a noun formed from a verb that means to "to seize, steal [hence the KJV's `robbery'], snatch, take away." Although its meaning has been much debated, there is a growing consensus that its probable sense leans toward something like either "a matter of grasping or seizing" or "something grasped for one's own personal advantage." In the first option the emphasis lies on the verbal side of the noun, on the idea of "seizing" as such. Thus Christ did not consider "equality with God" to consist of being "grasping" or "selfish"; rather he rejected this popular view of kingly power by pouring himself out for the sake of others. The alternative, which is probably preferable, is to see the word as a synonym of its cognate harpagma ("booty" or "prey"), which in idioms similar to Paul's denotes something like "a matter to be seized upon" in the sense of "taking advantage of" it ("he did not think he needed to take advantage of this equality with God," Bockmuehl 1997:114).
In either case, the clause comes out very much at the same point. Equality with God is something that was inherent to Christ in his preexistence; but he did not consider Godlikeness to consist in "grasping" or "seizing" or as "grasping it to his own advantage," which would be the normal expectation of lordly power—and the nadir of selfishness.
Third, Christ's selflessness for the sake of others expressed itself in his emptying himself by taking the "form" of a slave. Historically, far too much has been made of the verb "emptied himself," as though in becoming incarnate he literally "emptied himself" of something. However, just as harpagmos requires no object for Christ to "seize" but rather points to what is the opposite of God's character, so Christ did not empty himself of anything; he simply "emptied himself," poured himself out, as it were. In keeping with Paul's ordinary usage, this is metaphor, pure and simple. What modifies it is expressed in the phrase that follows; he "poured himself out by taking on the `form' of a slave."
Elsewhere this verb regularly means to become powerless or to be emptied of significance (hence the NIV's made himself nothing; cf. KJV, "made himself of no reputation"). Here it stands in direct antithesis to the "empty glory" of verse 3 and functions in the same way as the metaphorical "he became poor" in 2 Corinthians 8:9. Thus, as in the "not" side of this clause (v. 6b), we are still dealing with the character of God as revealed in the mindset and resulting activity of the Son of God. The concern is with divine selflessness: God is not an acquisitive being, grasping and seizing, but self-giving for the sake of others.
That this is Paul's intent is made certain by the two explanatory participial phrases that follow. The first explains the nature of Christ's emptying himself, how it was expressed in our human history: "by taking on the `form' of a slave" (NIV taking the very nature of a servant). From Paul's perspective this is how divine love manifests itself in its most characteristic and profuse expression. Christ entered our history not as kyrios ("Lord"), a name he acquires at his vindication (vv. 9-11), but as doulos ("slave"; see on 1:1), a person without advantages, rights or privileges, but in servanthood to all. All of this, surely, with an eye to verses 3-4. It is further important to note that crucifixion was reserved by the Romans either for insurrectionists or for recalcitrant slaves.
The second participial phrase simultaneously clarifies the first by elaboration and concludes the present sentence by paving the way for the next (v. 8). Together these two phrases give definition to Christ's "impoverishment." The phrase "in the form of a slave" comes first for rhetorical reasons—to sharpen the contrast with "in the form of God" and to set out the true nature of his incarnation. It thus reflects the "quality" of his incarnation. The second phrase indicates its "factual" side. Thus Christ came "in the form of a slave," that is, by being made in human likeness.
This leads, fourth, to the troubling word likeness (homoioma), which is most likely used because of Paul's belief (in common with the rest of the early church) that in becoming human Christ did not cease to be divine. This word allows for the ambiguity, emphasizing that he is similar to our humanity in some respects and dissimilar in others. The similarity lies with his full humanity; in his incarnation he was "like" in the sense of "the same as." The dissimilarity lies with his never ceasing to be "equal with God"; while "like" us in being fully identified with us, he was not "human" only. He was God living out a truly human life, all of which is safeguarded by this expression.
Thus, to put the sentence all together, in Christ Jesus God has shown his true nature; this is what it means for Christ to be "equal with God"—to pour himself out for the sake of others and to do so by taking the role of a slave. Hereby he not only reveals the character of God but also reveals what it means for us to be created in God's image, to bear his likeness and have his "mindset." It means taking the role of the slave for the sake of others—the contours of which are delineated in the next sentence.