This letter is vital to the church today, because as it was then the church has always had relationship troubles among it's people. The key verse to understanding why he writes is in vs. 2 of chapter 4. Two people having a falling out with other which is affecting the whole church. Well how do fix church squabbles? Paul spends the majority of this writing reminding them of the cross of Jesus and using the cross as a focus to fix all relationship problems. If we would only develop the mind of Christ on the cross when the trouble starts then it's very likely we will preseve our brotherhood instead of destroying it. The first step in good relations is to be cleat about what really matters in view of the cross and what really doesn't matter. This is a vital step in any relationship and jesus was our best example of this concerning us.

Philippians 1:9-10