This is the "kenosis" chapter of Philippians and is a call to humility. Humility is doing what Jesus did, that is to not use our position and power (whatever it is) for ourselves but to use our position and power in the service of others. Jesus "emptied himself" and did not count his position as "Son of God" as something to be used for his own advantage but took the servant's place and humbled himself for our sake and the salvation of the world.
Notice in 12 Paul says - "work out your own salvation with fear and trembling" - which would seem to to be saying that it is all up to us. Then, in 13, he says "it is God who works in you to will and to act in order to fulfill his good purpose." Which seems to be saying that it is all up to God. I love the Bible and St. Paul for these wonderful tensions. What I take this to mean is that I must do everything in my power to live the life that God has called me to live while AT THE SAME TIME recognizing that I am doing nothing for God, in actuality God, though the Holy Spirit, is actually working through me. So, on one level, it is all up to me but on the spiritual plane it is clearly all up to God. To put it another way, God and I are in partnership. God is working through me but the work through me is somewhat dependent on my willingness to allow God to work through me.
I am touched by how Paul speaks of his companions Timothy and Epaphroditus at the end of the chapter. When we think of the companions that God has placed in our lives, companions and co-workers in bringing the Kingdom of God, we should remind ourselves that they too are gifts from God and a blessing to each of us. Who are the co laborers in your life? I greatly appreciate the staff and leadership team at Christ Church -- my co-laborers in this ministry. They are dedicated hard working Christ followers worthy of our thanks and encouragement.