To discern how Paul uses morphe, please note the words of Moises Silva below:
If we stress the classical usage of this term [morphe], the technical sense of Aristotelian philosophy suggests itself: morphe, although not equivalent to ousia ("being, essence"), speaks of essential or characteristic attributes and thus is to be distinguished from schema (the changeable, external fashion). In a valuable essay on morphe and schema, [Lightfoot] argued along these lines and remarked that even in popular usage these respective meanings could be ascertained. The many references where morphe is used of physical appearance . . . make it difficult to maintain Lightfoot's precise distinction, though there is an important element of truth in his treatment. (Silva 113-114)
Upon closer examination, it becomes manifestly obvious that Phil 2:6-7 (by its use of morphe) does not unequivocally establish the essential deity of Christ. The employment of morphe in Philippians does not necessarily substantiate the teaching that Christ is God incarnate. To derive this conclusion from Phil 2:6 demonstrates a mistaken over-reliance on a single Greek term. Moises Silva offers further valuable comments along these lines as follows: "[Lightfoot's] claim that morphe (opposite schema) refers to unchangeable essence can be sustained by some references, but too many passages speak against it" (122). To verify this contention, Silva quotes Plato (Republic 380d) who inquires about God's ability to alter His "shape" (to autou eidos eis pollas morphas). The New Testament professor subsequently references Xenophon, Philo, Lucian and the fourth century writer, Libanius, who wrote: ouch ho tois theos tes morphen eoichos (123). All of these classical references indicate that morphe [in Phil 2:6] refers to an entity's outward appearance (not to a thing's intrinsic essence).
At this juncture, we must inform our readers that all of the foregoing information does not mean Silva denounces Trinitarianism; he surely does not concede that Phil 2:6 is dissonant with Trinitarian claims. His comments do help us to see, however, that one cannot base his or her belief in Christ's Deity on the mere occurrence of morphe vis-à-vis the preexistent Christ.