When Paul was writing to the Christians in Philippi, he was addressing the citizens of a unique city. Philippi had been established as an official Roman colony about a generation before Christ. As a Roman colony, Philippi governed by Roman law; citizens of the city had many special privileges both within the city and across the Empire; even the city’s layout resembled the Roman capital. In short, Philippi was a miniature Rome in northwestern Greece.
Citizens of Philippi were citizens of the Roman Empire, meant to spread the culture and influence of the western capital in their region to the east. Having Philippi as an official colony some 600 miles to the east was a valuable asset to Rome in having a footprint in another, distant portion of the Empire.
So when Paul writes in Philippians 3:20 that the Philippian Christians have their citizenship in heaven, he is making a profound statement, loaded with meaning for he believers in Philippi. The Philippian Christians would have understood: citizenship in heaven was not about simply enduring this world and longing to get into heaven when they died.
Citizenship in heaven was all about being an outpost of heaven in a distant land. It was about being a colony, meant to export the culture, values, justice, and resources of the capital. The Philippians would have understood Paul’s imagery: their citizenship in heaven meant they had a job to do in this world.