Commenting on Philippians 2:6 from the TNIV, Gordon Fee clarify a reality of the first century Christians.

What the earliest followers of Christ had come to believe, of course, on the basis of his resurrection and ascension, was 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.  It is this understanding that (correctly) lies behind the TNIV’s “in very nature God.”  And it is this singular reality, lying in the emphatic first position as the phrase does, that gives potency to what follows and therefore to the whole. – Gordon Fee, “Pauline Christology” p379