Paul’s command in Philippians 2 calls on Christians to have the same mindset that “was also in Christ Jesus who, although existing (huparchon) in the form of God,” did not exploit or misuse this authority.

The Greek word huparchon is a present active participle with Christ as its antecedent, i.e., the state of “existing in the form of God” was while he was on earth and not a reference to before his birth. Yet, many mistranslate the Greek in the past tense (i.e., “he was”) because the goal is to read into the text the so-called preexistence of Christ. 

This is in light of the fact that “Judaism has never known anything of a preexistence regarding the Messiah prior to his birth. The dominance of the idea in any Jewish circle whatever cannot seriously be upheld.” (Dalman, The Word of Jesus)  

So it’s clear from both the context of the passage in Philippians 2:5-11 (ethical vs theological) and in particular v. 5 that it was the human, historical Messiah Paul had in mind. And not a description of some preexistent figure who “assumed” or “took on flesh” to become a human, the anointed one of God.