Work Out Your Salvation (Philippians 1:6; 2:12,13; COD 3,4; 14-16)

We know that we are saved by grace that is applied by the will of God. God is the one who chooses his people, empowers his people, and will preserve his people until the end. If this is true, then why does Paul tell us to work out our salvation with fear and trembling? Does God just start the work and then we need to finish the work?