Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, (ESV)
Every time you see a "therefore" in Scripture you need to look and see what it is "there for." The Apostle Paul has just finished a powerful section describing the humility of Christ. He laid aside the glory that was rightfully his and humbled himself by becoming a man, and even demonstrated the ultimate obedience to God by being crucified for our sins. Therefore . . .
Therefore, you have a responsibility as well. You are to work out your own salvation with fear and trembling. Paul gives this command in response to what Christ has done, and he appeals to their obedience which they have already demonstrated. As this is a command for the Philippian believers it is also a command for us.
So, what does it mean for us to work out our own salvation? We know that we are saved by the grace of God and not by works (Ephesians 2:8-9). Paul is not saying that we should work for our salvation, but rather that we should work out our salvation. In other words, salvation is not a one-time event in our lives, but rather a lifetime of growing in that relationship with our wonderful Savior.
This working out of our salvation would include our prayer life, our study of the Word of God, our fellowship with other believers, attending corporate worship, sharing our faith with others, and allowing God to lead us in every aspect of our lives. And we are to do this with fear and trembling. In other words, we are to take this work seriously.
How well are you working out your own salvation? Do you work it out with fear and trembling? Or is your salvation an occasional afterthought to the busyness of life? How can that improve?