By the time Paul writes Philippians, the events recorded in the book of Acts have all come to pass. Of course, that means that the Messiah has come—living righteously, dying sacrificially, and rising victoriously. He has sent his followers into the world to proclaim the gospel, make disciples of the nations, and plant local churches. The book of Acts records that: 30 or so years of the gospel spreading and churches forming.
Thus, it is clear that the Kingdom of God has arrived but not yet in its fullness. It is both now and not yet fully come. Christians are redeemed but must keep pressing on: standing firm in their confession, working out their salvation in Christian growth, resisting false teachers, embracing suffering and persecution, holding out the gospel of hope to the world, and committing to live out Christ’s love and humility with each other in the church.
Paul wrote several letters to churches and individuals (Romans–Philemon). From one angle, they all share a general purpose—to encourage and equip Christians for the advancement of the gospel (Phil. 1:12) and progress in the faith (Phil.1:25). But, from another angle, each letter has its own unique purpose, context, background, and emphases. One distinctive of Philippians is its emphasis on partnership or sharing. The Philippian Christians share the gospel and the gospel mission with Paul, as they do among themselves as a church.