Paul’s command to “work out our salvation” might come as a surprise (v. 12). Paul is so adamant elsewhere that we are not saved by works, but rather grace through faith. In fact, this verse has been the source of a great deal of post-Reformation theological discussion. But Paul does is not talking about working for your salvation, or working to keep your salvation, but rather to continue obeying God, as the members of the church at Philippi are already doing. Paul assumes the church is already obedient to God’s commands and they are already following the model for obedience is Jesus (vv. 5-11) and Paul (v. 17).
To “work out” (κατεργάζομαι) often refers to producing something agriculturally, perhaps “cultivate” fruit is a possible source for this metaphor (LXX Deut 28:39, Ezek 34:3, 36:9, Odes 1:17). That is the point of the word in Romans 7:17-18, sin is like a seed that produces an evil behavior. People talk about “cultivating a relationship” in business. This means making contacts and doing little things that will eventually result in a sale. The goal is a sale, but there are dozens of smaller contacts along the way that build up to closing the deal.
Salvation is a completed fact when someone accepts Christ as their savior, they are “justified” before God, but (obviously) they are not yet sinless nor have they arrived in Heaven yet. “Working out one’s salvation” can be understood as cultivating what God has already done so that it yields fruit at the appoint time.
The believer is to cultivate their salvation with “fear and trembling.” Paul used this phrase in Eph 6:5 (“slaves, obey your masters with fear and trembling”) and 2 Cor 7:15, referring to how that church received Titus. In both cases there is a real fear of punishment for wrongdoing. The Old Testament occasionally describes salvation and service of the Lord in terms of fear (Ps 2:11, for example). There are a number of examples, however, of dread falling in the nations when they encounter God (Exod 15:16, Isa 19:16).
Paul has already mentioned the church’s fear of oppression from the culture in 1:28, it is possible this fear and trembling refers to the dread the members of the church have as they face ridicule and pressure to conform to Roman culture.
Even though Paul says we are to “work out” our salvation, it is God who is doing the work in the life of the believer (v. 13). The believer is not left to their own to cultivate their salvation, it is in fact God who “wills and works” in us. God is the one who is at work in the life of the believer, enabling the believer to grow spiritually. While he does not mention the Holy Spirit in this verse, this is exactly the same sort of thing we read in Galatians or Romans, that one is enabled by the Holy Spirit to do the will of God (Rom 12:1-2, for example).
Remember the junior high science fair? I know a Middle School principal who called the Science Fair “parent’s projects” since most of the winning projects were largely the result of dads with power tools and too much time on their hands. A parent could do all the work, but they should not be proud of the child for winning the award, and the child will not develop the maturity and skills needed to succeed in life. But if a parent helps the child and provide them what they need to succeed, then the parent can be legitimately proud when their child wins the award and the child will grow from the experience (even if they lose to the kid whose parents cheated).
In a very similar way, God provides the believer with all they need to succeed in their spiritual life, to “cultivate their salvation” and produce real fruit that will make God proud when he judges our works at the resurrection. We are more mature for the struggles we endured during that cultivation.