The word μορφῇ means "form, outward appearance, [or] shape"<sup>1</sup> (occurring in the dative in this context following the preposition ἐν). To be very blunt, translating this as "nature" (as the NIV does) is a poor translation choice. Discussion of God's nature is theologically charged and thus using "nature" in this context could be misleading. The NET translators explain the usage of μορφῇ in Philippians 2:6 by saying:

> The Greek term translated *form* indicates a correspondence with
> reality. Thus the meaning of this phrase is that Christ was truly
> God.<sup>2</sup>

The next verse uses the same word (but in the accusative) in the phrase μορφὴν δούλου λαβών ("taking [the] form of a slave"). Jesus truly was God, and also he truly was a servant. For an in depth understanding of the usage of μορφῇ both in biblical and extra-biblical writings I have copied a lexical entry for you:

> **μορφή, ῆς, ἡ** (Hom.+) **form, outward appearance, shape** gener. of bodily form 1 Cl 39:3; ApcPt 4:13 (Job 4:16; ApcEsdr 4:14 p. 28, 16
> Tdf.; SJCh 78, 13). Of the shape or form of statues (Jos., Vi. 65;
> Iren. 1, 8, 1 [Harv. I 67, 11]) Dg 2:3. Of appearances in visions,
> etc., similar to persons (Callisthenes [IV B.C.]: 124 Fgm. 13 p. 644,
> 32 Jac. [in Athen. 10, 75, 452b] Λιμὸς ἔχων γυναικὸς μορφήν; Diod S 3,
> 31, 4 ἐν μορφαῖς ἀνθρώπων; TestAbr A 16 p. 97, 11 [Stone p. 42]
> ἀρχαγγέλου μορφὴν περικείμενος; Jos., Ant. 5, 213 a messenger fr.
> heaven νεανίσκου μορφῇ): of God’s assembly, the church Hv 3, 10, 2; 9;
> 3, 11, 1; 3, 13, 1; Hs 9, 1, 1; of the angel of repentance ἡ μ. αὐτοῦ
> ἠλλοιώθη his appearance had changed m 12, 4, 1. Of Christ (ἐν μ.
> ἀνθρώπου TestBenj 10:7; Just., D. 61, 1; Tat. 2, 1; Hippol., Ref. 5,
> 16, 10. Cp. Did., Gen. 56, 18; of deities ἐν ἀνθρωπίνῃ μορφῇ: Iambl.,
> Vi. Pyth. 6, 30; cp. Philo, Abr. 118) μορφὴν δούλου λαβών he took on
> the form of a slave=expression of servility Phil 2:7 (w. σχῆμα as
> Aristot., Cat. 10a, 11f, PA 640b, 30–36). This is in contrast to
> expression of divinity in the preëxistent Christ: ἐν μ. θεοῦ ὑπάρχων
> although he was in the form of God (cp. OGI 383, 40f: Antiochus’ body
> is the framework for his μ. or essential identity as a descendant of
> divinities; sim. human fragility [Phil 2:7] becomes the supporting
> framework for Christ’s servility and therefore of his κένωσις [on the
> appearance one projects cp. the epitaph EpigrAnat 17, ’91, 156, no. 3,
> 5–8]; on μορφὴ θεοῦ cp. Orig., C. Cels. 7, 66, 21; Pla., Rep. 2, 380d;
> 381bc; X., Mem. 4, 3, 13; Diog. L. 1, 10 the Egyptians say μὴ εἰδέναι
> τοῦ θεοῦ μορφήν; Philo, Leg. ad Gai. 80; 110; Jos., C. Ap. 2, 190;
> Just., A I, 9, 1; PGM 7, 563; 13, 272; 584.—Rtzst., Mysterienrel.3
> 357f) Phil 2:6. The risen Christ ἐφανερώθη ἐν ἑτέρᾳ μορφῇ appeared in
> a different form Mk 16:12 (of the transfiguration of Jesus: ἔδειξεν
> ἡμῖν τὴν ἔνδοξον μορφὴν ἑαυτοῦ Orig., C. Cels. 6, 68, 23). For lit. s.
> on ἁρπαγμός and κενόω 1b; RMartin, ET 70, ’59, 183f.—DSteenberg, The
> Case against the Synonymity of μορφή and εἰκών: JSNT 34, ’88, 77–86;
> GStroumsa, HTR 76, ’83, 269–88 (Semitic background).—DELG. Schmidt,
> Syn. IV 345–60. M-M. EDNT. TW. Spicq. Sv.<sup>3</sup>

Concerning the usage of μορφή in both passages, in v. 6 "being" (ὑπάρχων) in the form of God was natural (he was 'in the form of God'), while in v. 7 'taking' (λαβών) the form of a slave was an active choice.

<h3>Sources</h3>
<sup>1</sup> William Arndt, Frederick W. Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature, 3rd ed. (Chicago: University of Chicago Press, 2000), 659.

<sup>2</sup> Biblical Studies Press, The NET Bible First Edition Notes (Biblical Studies Press, 2006), Php 2:6.

<sup>3</sup> Arndt, Danker, and Bauer, 659.