If salvation is entirely a free gift of grace from God, do we need to do anything after receiving it? Is grace opposed to effort? In this enigmatic passage we see how Paul views the interplay between our work and God's work - "work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for his good pleasure."
But how does this working take shape? Through embracing the path of self-denial to obey Jesus and love others without grumbling or disputing. Because it is so natural for humans to grumble about any and everything, those who do not have the opportunity to shine as lights in the world, demonstrating that they operate according to a different and more joyful perspective.