John Piper introduces a thought-provoking article this way:
One of the most influential passages in the Bible that God used to open my mind to his sovereignty over my will is Philippians 2:12-13.
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.
So my working and willing are necessary. They are real. But they are not first or ultimately decisive. God’s willing and working is decisively under and in my willing and working. The word “for” is crucial. I work because he is working in me. I will, because he is willing in me.
(Read the article here.)