Philippians 2:12, 13–“Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, 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 why fear and trembling in sanctification?  Dr. John Piper unpacks this question in his sermon, “I Act the Miracle.”  An excerpt from that sermon can be found here.