for it is God who works in you, both to will and to work for his good pleasure. (ESV)
This verse proves why we must always look at Scripture in its context. If we were just to look at this verse without reading the preceding verse we might completely miss what the Apostle Paul is trying to tell us. This verse implies that God is doing the work in us, so there must not be much for us to do. To avoid the erroneous interpretation, let's look at both of those verses together:
 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. (ESV)
We are to work. In fact, we are to work out our salvation with a sober and serious effort before God. Yet, where do we get the power to work? It is God's power working through us! I am so encouraged to know that God does not call us to do something that he does not also give us the power to do. I have heard it said this way: "God does not call the qualified, he qualifies the called."
There are many things that we will find very difficult when it comes to working out our own salvation. We will have to endure persecution. We will have to love those who hate us. We will have to forgive things that are unforgivable. We do not give up. We work out those aspects of our new salvation life. Our wonderful Savior will give us the strength that we need. He is working through us.
What is the most difficult area of your salvation that you need to work out today? Ask the Savior to empower you by his Holy Spirit to accomplish what he is calling you to do. He will!