Paul begins a new section at the beginning of Chapter 3. We know that because his hinge phrase throughout the book always begins with rejoice! So in Philippians 3:1 he says, “Finally, my brothers, rejoice in the Lord.” 
The phrase beginning with “Finally” has been used as an occasion for humor, most at the expense of us preachers. One little boy whispered to his father during the sermon, “What does the past mean when he says ‘finally’ dad?” His father answers, “Absolutely nothing, son.” Paul uses the word to introduce a hinge idea regarding rejoicing again in the middle of his letter. In other words he still has some more to say. He goes on for two more chapters. But the word really doesn’t mean the same thing we mean when we say “finally.” It’s actually a transitional marker identifying the movement to a new subject.
The interesting thing here is that if his readers were looking for something entirely different they would be disappointed. He’s still talking about the joy we experience in living the Christian life together. He’s building up this theme of joy over and over in this epistle. He will come to a great crescendo in Chapter 4 when he says, “rejoice in the Lord always, again I say, rejoice!” He has been talking about joy in various ways in the first two chapters, but this is the first time he connects it with the little phrase, “in the Lord.” As we all know we can’t always rejoice in our circumstances because they are not always what we’d choose for ourselves. We all face trials and struggles in life and during those times it seems almost ridiculous to be told to “rejoice.”
We can always “rejoice in the Lord.” We know that He conquered sin and death and in the end we will all be victorious over the sicknesses, sorrows, and diseases of the world. So, like Job, we know that God is good all the time and that even in hard times we can still rejoice. The better we know our “Good God” the easier it is to “rejoice in the Lord.” He enables us to live above our circumstances not under them. When we come to grips with the love and goodness of our God even in the times of trials it helps make everything else so insignificant. Over the years of my Christian life I’ve let myself become a victim of my circumstances and have sacrificed my joy to the demands and pressures of the world. Yet when I come to my senses, I realize the greatness and goodness of our God and can put the “light and momentary” afflictions of this life into the divine perspective which helps maintain my joy in the Lord.