Paul wrote his letter to the Philippians to acknowledge the gifts from them (4:10-19). Along the way, he also urges two individuals to be reconciled to one another (4:1-3). One of the key features of this letter is the so-called hymn of Christ (2:5-11). This, along with John 1:1-18, is one of the most exalted statements about Jesus’ divinity and pre-existence in the New Testament. Thus, Phil 2:5-11 is a brief account of the gospel.
However, the letter is not primarily about doctrine. Rather, Paul also focuses on such Christian virtues like humility and unity (1:27-2:4), knowing Christ (3:8-11), and joy (1:4, 25; 2:2, 29; 4:1). Paul's main burden in writing is to describe what the gospel looks like in real life.