The sense in which we are to work out our salvation in fear and trembling is twofold. First, the Greek verb rendered “work out” means "to continually work to bring something to completion or fruition." We do this by actively pursuing obedience in the process of sanctification, which Paul explains further in the next chapter of Philippians. He describes himself as “straining” and “pressing on” toward the goal of Christ likeness (Philippians 3:13-14). The “trembling” he experiences is the attitude Christians are to have in pursuing this goal—a healthy fear of offending God through disobedience and an awe and respect for His majesty and holiness.
"Trembling" can
also refer to a shaking due to weakness, but this is a weakness of higher
purpose, one that brings us to a state of dependency on God. If you have ever watched the Olympic games, you see athletes working so hard their muscles are trembling with strain. I wonder how many of our spiritual muscles actually strain? Obedience and
submission to the God we revere and respect is our “reasonable service” (Romans 12:1-2) and
brings great joy. 


