Therefore, my dear friends, as you have always obeyed—not only in my presence, but now much more in my absence—continue to work out your salvation with fear and trembling, for it is God who works in you to will and to act according to his good purpose.
It has always struck me as an odd turn of phrase that Paul uses, "work out your salvation with fear and trembling." I've never really quite got to grips with it. Surely Jesus set me free from fear, surely as a follower of Christ there is little to work out and nothing over which I need to tremble. So what could Paul possibly be getting at?
The obvious analysis of the structure of the sentence (you had noticed that verses 12 ad 13 are one sentence I'm sure) is that continue to work out is synonymous with obeyed, and sits in the context of God's continuing work in our lives. When it comes to fear and trembling it seems most likely that Paul is talking about an attitude of faith. It's an appropriate attitude. When the people came to the mountain where they were to meet God in the exodus story, they approached with fear and trembling. They came with reverence towards the God who had demonstrated his power.
When we work out our salvation with fear and trembling, it is not based on the worst case scenario of what might go wrong or how easily we might lose our connection with God and ultimately our eternal security. That's the wrong kind of fear. We come knowing that God is at work in us and desperate to walk in light of this truth. We want so much to live a life worthy of the gospel (1:27).
The only thing we have to offer God is our full devotion, our total obedience. Nothing more is needed, nothing less will do.