Fulfilling the example of Christ in our own lives does not finally depend on us. God pours into us the ability to trust in him and his purposes: “it is God who works in you …” Paul has already mentioned God’s sovereignty as the ground of his confidence and joy, back in chapter 1: “being confident of this, that he who began a good work in you will carry it on to completion until the day of Christ Jesus” (1:6). Now, however, Paul particularly exhorts them to work out their salvation while they rest in God’s sovereignty—“… to work and to act according to his good purpose.”
When Christians discover the doctrine of God’s sovereignty, we discover great joy and assurance. We discover assurance of his victory: “He who began a good work in you will carry it on to completion.” And we discover the joy of being called to participate with him in that victory: “it is God who works in you to will and to act according to his good purpose.” Paul provides an excellent example of this joy when he writes that in all his prayers for the Philippians, “I always pray with joy” (1:4), even though he and the Philippians are both in difficult, even dangerous, situations.
These difficult and dangerous situations, strange as it may seem, provide opportunity for the working out of God’s plan of salvation. The opposition the Philippians face is a sign to them that “you will be saved—and that by God. For it has been granted to you on behalf of Christ not only to believe on him, but also to suffer for him” (1:28–29). So when he directly exhorts them, “work out your salvation,” he means that they, like Jesus himself, have been called—indeed, granted!—to persevere in not living for themselves but for others.
Dever, Mark, and John F. MacArthur Jr. 2005. “The Message of Philippians: Humility.” In The Message of the New Testament: Promises Kept, 272. Wheaton, IL: Crossway.