In that the Church is the ecclesia of God, the assembly of ruling citizens of the City of God, believers are on solid ground to regard their nation-state citizenships as secondary to their primary citizenship in the Kingdom of God. The Kingdom of God is not an idea; it is a meta-culture with its own calendar, music, holydays (which most people spell as “holidays”), heroes, history, flag, language, rituals, ceremonies, and so forth. The Kingdom of God doesn’t just have a president or a prime minister, but an Emperor, the Lord Jesus Christ, the King of Kings and Lord of Lords (which is what an Emperor is). The Apostle Paul was a Roman citizen who leveraged that citizenship on behalf of his primary citizenship in the Kingdom of God. The charism of Basileia is one that instills this sense of kingdom citizenship.
Also see Ecclesial City.