Years ago, when I was in college, I participated in a Bible study group for college students. The object lesson I'm sharing here is not mine; I first heard it from Lenny, the leader of that Bible study group. I found it helpful, so I thought I'd share it here.
Imagine that you get a phone call from a friend who says he has a gift to give you. Curious, you ask him what it is. He tells you that he has received a van Gogh painting, and he would like to pass it on to you. You are stunned, knowing that any van Gogh painting is worth millions of dollars.
Your friend tells you that you need to make arrangements to come pick up the painting and bring it home. Imagine how you feel at this moment. How are you going to collect the painting? Will you drive your car there and pick it up yourself? Will you hire a moving van? Or maybe even an armored truck? And what about once you've received the painting? What will you do with it? What kinds of security systems will you have to install in your house? What about UV protection for the artwork?
As you pick up the painting, and as you install it in your home, you approach the entire process with "fear and trembling." Not because you are afraid that your friend will take back his gift, but because you understand how precious and valuable it is, and how easily damaged it is. It is a treasured possession, and you want to guard it and protect it.
In Philippians 2, Paul writes:
Paul is not telling us that our salvation is ours to earn. No, like that van Gogh painting, it is a gift beyond value or belief. But now that it is ours, we treat it with all the care and regard that a priceless gift deserves. We understand that the gift of God's grace is fully ours, but with that gift comes a grave responsibility.
And in the midst of this, Paul offers this reassurance -- you're not on your own. As he had already promised in Philippians 1:6, God is, and will be at work in you.
As an added note to Lenny's object lesson, it's interesting to note that the phrase "work out" in this verse comes from a compound Greek word: katergazomai. Kata has a variety of meanings, so any compound word built from it also may have many possible meanings. Kata has these likely meanings in this verse: "toward the completion of," "in the direction of," or "according to." Gazomai simply means "work." Thus, we could alternately translate that phrase as "work according to your salvation," "work toward the completion of your salvation," or "work in the direction of your salvation." That fits nicely with the next verse which assures us that God is working in the same direction!
Not work for it, but work according to it, and toward its fulfillment at the day of the Lord. My life now, in this world, should be aimed toward that future day of glory.