Philippians 2:12 is a bane to those who feel insecure in their faith, and a boon to those who want to keep it that way. "Work out your own salvation with fear and trembling", writes the blessed Apostle. "Which means you can lose your salvation", some people add.
But is this really what the verse is saying? In truth, no. Indeed, if it was, there would be much clearer ways to say it. By contrast, the Bible is clear to the point of excess that salvation does not require working for. Just as you cannot work to gain salvation, you also cannot work to keep it. In truth, salvation is based entirely on the work of Christ, which, incidentally, is affirmed just one chapter before our "problem" verse. "I thank my God upon every remembrance of you, always in every prayer of mine making request for you all with joy, for your fellowship in the gospel from the first day until now, being confident of this very thing, that He who has begun a good work in you will complete it until the day of Jesus Christ;" (Philippians 1:3-6).
So if Paul is not saying "work or go to Hell", what is he actually saying? In the English, one very simple word helps us figure that out: Work out your own salvation with fear and trembling. Interestingly, we have a similar phrase in common use: live out.
Here, Paul is actually talking about consistency. He makes this abundantly clear throughout Philippians, and he actually praises the church at Philippi for how well they've already been doing this. "Therefore, my beloved, as you have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling; for it is God who works in you both to will and to do for His good pleasure."
And so we see that Philippians 2:12 is not some sort of biblical anomaly. It is not a rare instance of God changing His mind and adding works to salvation. Rather, it is an exhortation to the saved to actually act saved, knowing the kind of God we were saved by. And in the rest of the book, Paul describes quite graphically what that is supposed to look like. He talks about how Christians should be like minded, and should do nothing out of selfish ambition. He talks about how we should esteem each other greater than ourselves, looking out not only for our own interests, but for each others' interests as well. Then he moves on to using Christ as the example. Though being God Himself, He didn't use His equality with God for His own sake, but for the good of others. In a sense, this hearkens back to the dispute between the Apostles in Luke 22:24-30, wherein Jesus says that the greater should serve the lesser, as indeed Jesus, though indisputably greater, serves lowly humans.
And so we see what working out our salvation means. In effect, it just means practice what you preach. You preach a holy God? Ok, be holy. You preach that sin brought death? Then repent. You preach that God predestined us to good works? What's your excuse for laziness? The long and short of the book of Philippians is if you're going to be a Christian, be a Christian. There's no point in getting saved if you're not going to act saved.