The Book of Philippians is about the Gospel. It’s not quite an exposition of the doctrinal content of the Gospel, like Romans is. And it’s not quite a defense of the Gospel in the face of heresy, like Galatians is. Philippians is more about the implications the Gospel has on the various aspects of our lives as believers. It’s about how we are to live in light of the Gospel. The thesis verse of the letter is Philippians 1:27, in which Paul commands his dear friends to “conduct yourselves in a manner worthy of the Gospel of Christ.”
And in the context of the opposition the Philippians were facing because of their commitment to Christ, living in a manner worthy of the Gospel chiefly involved being united with one another. If the people of God are to have any hope of standing firm in the face of opposition (1:27b)—if they are to have any hope of propagating this Gospel of repentance and faith in Jesus Christ amidst a hostile society (1:27c)—they will need to be unified. And so in the opening verses of chapter 2, Paul calls them to unity—as well as to the humility without which that unity won’t ever be achieved.
Then, in verses 5 to 11, he pens perhaps the loftiest and most precise Christology anywhere else in Scripture. He speaks in detail about the Lord’s pre-existence as the eternal Son of God, the mystery of the incarnation and the kenosis, the hypostatic union—Christ’s being fully God and fully man, having two natures “without confusion, without change, without division, and without separation” bound up in a single person. He speaks of the mystery of the sinless God-man dying: God cursed and forsaken by God. And he speaks of His glorious resurrection from the dead, ascension into heaven, and His exaltation to the Father’s right hand.