The difficulty you may be having in understanding the apostle’s meaning may be due to the fact that we often conceive of salvation too narrowly. “Salvation” is a comprehensive term. We tend to think of only one aspect of salvation as if it were the whole of it. We tend to use the term exclusively of the moment of our conversion. We tell people, “I was saved when I was 12 years old.” Or, “I was saved when I lived in Wichita.” Etc. When we say things like this, we are thinking of the moment of our conversion and equating it with “salvation.” There is nothing wrong with this as long as we remember that there is more to our salvation than this initial conversion experience when we are justified, or put into a right relationship with God.
The Bible uses the word “salvation” to refer, not only to conversion, but also to everything that follows from it. According to the language of Scripture, we have been saved, we are being saved, and we shall be saved. In other words, there is a past, present, and future element to our salvation. Note the past, present and future tenses in the following verses relative to our salvation.
Past tense: “He saved us, not because of works done by us in righteousness, but according to his own mercy” (Titus 3:5).
Present tense: “For the word of the cross is folly to those who are perishing, but to us who are being saved it is the power of God” (1 Cor. ).
Future tense: “Since, therefore, we have now been justified by his blood, much more shall we be saved by him from the wrath of God” (Rom. 5:9).
At conversion we are justified. Justification is an act of God’s pure grace by which he pardons all our sins and accepts us as righteous in his sight through the merits of Jesus’ blood, accepting his death in the place of our punishment. It is a one time, once for all work of God in the life of the believer that takes place at the moment of our conversion. This is the past tense of our salvation, and is usually what we have in mind when we speak of “salvation.”
But following justification comes sanctification; and whereas justification is an instantaneous work that God does for us, sanctification is a progressive work that God does in us. In justification God reckons us to be righteous; in sanctification, he makes us so in practice. By the working of his grace in us, he brings us into greater and greater conformity to his will. We are justified at the moment of our conversion, and it is a completed work. But sanctification begins with conversion and continues till the moment of death. This is the present tense of our salvation.
At death, the Christian is “glorified.” That is, he is finally and fully delivered from sin and all its consequences. He is fully established in righteousness and sin is no longer even a possibility. In this life we must contend with the world, the flesh and the devil in our fight to live righteously. But in heaven we will be delivered from all our enemies and the fight will be over. Our salvation will be complete. This is the future tense of our salvation.
We have been saved (justified), we are in the process of being saved (sanctified), and one day will be saved (glorification).
When Paul said, “work out your own salvation with fear and trembling,” he was referring to the process of sanctification—working out in practice what God is working in us. The very next verse says, “For it is God who works in you, both to will and to work for his good pleasure” (Phil. ). This helps us to understand what he means in the verse our questioner referred to. God is graciously at work in us moving our will to will what he wills, and to do what he would have us do. This is a work that we are to yield to and cooperate with. But why with “fear and trembling”? Because, as Scripture says, “our God is a consuming fire” (Heb. ). Peter said, “If you call on him as Father who judges impartially according to each one’s deeds, conduct yourselves with fear throughout the time of your exile” (1 Pet. 1:17). God is holy, and he impartially judges with temporal judgments both saints and sinners. In fact, Peter would say just a little later, “It is time for judgment to begin at the household of God; and if it begins with us, what will be the outcome for those who do not obey the gospel of God?” (1 Pet. 4:17).