Paul writes his letter to the Philippians referencing two related forms of persecution. The Christians of Philippi are experiencing persecution by the Roman authorities who govern the Philippi. Paul himself is experiencing persecution by the Roman authorities (the Roman emperor's praetorian guard) in Rome. Roman persecution is thus the fundamental context for his letter. Paul's most basic premise in this letter is to hold high the slave, Jesus Christ!
In perhaps the most moving passage he ever wrote (Phil 2:6-11) Paul delineates that Jesus did not count equality with God something to be grasped at but rather empties himself taking the form of a slave and embracing crucifixion, the slave's form of death. Accordingly. God has raised Jesus to the highest place and has caused all on the earth, above the earth, and under the earther to bow at the name of Jesus and to proclaim that Jesus alone is Lord to the glory of God the Father.
Very significantly, Paul affirms that Christian slaves are ennobled by Jesus initiatives. Because these slaves now join the other Christian citizens in having "citizenship in heaven," Paul's letter constitutes a radical threat to the Roman imperial authorities in this way as well.