for it is God who is at work among you, both to will and to work, for the sake of his good pleasure.
In context
11and every tongue confess that Jesus Christ is Lord, to the glory of God the Father. 12So then, my beloved, just as you have always obeyed — not only when I was present, but now much more in my absence — work out your own salvation with fear and trembling; 13for it is God who is at work among you, both to will and to work, for the sake of his good pleasure. 14Do everything without grumbling or arguing, 15so that you may be blameless and innocent, children of God without blemish in the midst of a crooked and twisted generation, among whom you shine like lights in the world,