Here Paul shows the unique cooperation which occurs in the Christian life between us and God — we work out our salvation while God simultaneously works out our salvation. We believe while He provides the faith. We love because He first loves us. Without God, there is no salvation.
In Eden, God has already decided the means for our salvation — He even warns the serpent that Eve’s child will crush Satan’s head while Satan will only bruise the heel of her Son. God has a plan — a plan He demonstrates repeatedly throughout history until the birth of His One and Only Son, Jesus Christ.
Who works out our salvation “in fear and trembling?” The person who feels and displays the greatest “fear and trembling” for our salvation is Jesus as He kneels in the garden of Gethsemane wrestling with His dread of suffering, death and separation from His Father. What could be more terrifying? Nothing we experience can compare to the fear Jesus feels that night and on that cross. Whose purpose is being fulfilled? God the Father plans everything out to the last detail. He knows all; nothing takes Him by surprise.
For our part, Paul exhorts — “Show yourselves guileless and above reproach, faultless children of God in a warped and crooked generation, in which you shine like stars in a dark world and proffer the Word of Life.” (Philippians 2:14-16)