This scripture has the unfortunate combination of being theologically significant while also being hard to translate. Verses 5-7 in the King James version read as follows:
5 "Let this mind be in you, which was also in Christ Jesus: 6 who, being in the form of God, thought it not robbery to be equal with God: 7 but made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men"
Which implies that Jesus was equal with God. Yet Jesus himself said: "My Father is greater than I" (John 14:28). Trinitarians wriggle around that in their usual pagan way but it is a pretty blatant contradiction.
And the whole interpretation depends heavily on the meaning of one Greek word: "harpagmon". It is mostly translated as "robbery" but it is a very rare word so firming up the meaning of it is difficult. I have a list of 7 different translations of it from 7 different Bible scholars.
Thanks to the immense resources for Bible study that American Christians have put online, I was even able to check the translation of "harpagmon" in the Wycliffe Bible, the first translation of the Bible done in a vernacular language -- albeit in Middle English. Its version is:
"which when he was in the form of God, deemed not raven, that himself were even to God"
But "raven" there is metaphorically equivalent to "robbery" in modern or Early New English so adds nothing to our present enquiries.
And, even more importantly, the KJV/Wycliffe translation "the form of God" above is misleading. The original Greek is "morphe theou", literally "of god form". The definite article is not used in the Greek so it is not the central God of the Christians that is being referred to at all. The text simply says that Jesus was godlike or of divine essence -- "a god", if you like. There are many spirit beings in Heaven so it is implied that Jesus was simply one of them, not the big boss over all.
Even without relying on fine points of Greek grammar, however, it should be clear that when Paul said Jesus was "morphe theos" he was in fact making clear that Jesus was NOT God. Jesus was simply in the form or shape of a god. If Paul had wanted to say that Jesus WAS God ("ho theos") there was nothing to stop him. But he was careful to claim only that Jesus had something in common with God -- his form or shape, probably meaning only that he was a spirit being. That Paul did believe in spirit beings we read at some length in 1 Corinthians 15.
Given all that, I think the meaning of the text as a whole is quite clear. I would translate it as: "who, although being of divine form did not try to hang on to that but [became a man]"
So I translate "harpagmon" as "hang on to", which makes perfect sense of the passage as a whole. I interpret "harpagmon" in context, in other words. And I am not going far out on a limb in doing that. "something to cling to", "something to hold on to" are used by other translators. See here.
So there is no contradiction with John 14:28. The humility of a spirit being becoming flesh is simply being pointed out.