This book is mostly about how God is working through the church. Jesus is the head of the church, and the unique thing about the church is that it is not only a group of Jews, but it is Jews and Gentiles together (not Gentiles who became Jews by converting). In the first three chapters, Paul mostly explains what the church is and the great privileges that believers have. In the second three chapters, Paul tells the believers how they should live. Thus there is a lot of good, practical wisdom here.
The verse that most closely captures the argument of the book is 4:1: “As a prisoner for the Lord, then, I urge you to live a life worthy [chapters 4-6] of the calling you have received [chapters 1-3].” Located in between the two halves, this verse serves as the book’s “hinge.”