Oscar Cullmann on Philippians 2:6ff (Christology of the NT, Page 178)

In answer to the much discussed question whether the verb ἐκένωσεν ἑαυτὸν (he emptied himself) refers to the pre-existent or the incarnate Christ, we may say that this text probably includes both ideas: the idea that the Man became a Man; and the idea, mentioned in v. 8, that he assumed the role of the ebed Yahweh. According to both ideas the Son of Man proved his obedience (ὑπήκοος, v. 8) in contrast to Adam. This obedience is the determining factor, for Adam's sin consisted precisely in his disobedience. This leads us back again to Rom. 5.19, which also characterizes Adam by his disobedience and the Son of Man Jesus by his obedience. Jesus' likeness to God is revealed precisely in his obedience, which expresses itself in a double way: in his becoming man in the flesh, and in his humbling himself unto death (his accepting the role of the ebed Yahweh). Jesus must assume the form of fallen man in order to take the μορφή δούλου.
     The phrase which follows in v. 7, being born 'in the likeness of men' (ἐν ὁμοιώματι ἀνθρώπων), shows that Jesus entered completely into fallen humanity. This understanding of ὁμοίωμα is thoroughly attested.2 The next sentence accentuates Jesus' complete assumption of incarnate human nature: this is how the Man became man. He who by nature was the only God-man, who deserved the designation by bearing the image of God, became man in fallen flesh through obedience, in which he proved himself precisely the Heavenly Man, and by which he accomplished his atoning work.