Therefore, my dear friends, as you have always obeyed—not only in my presence, but now much more in my absence—continue to work out your salvation with fear and trembling, for it is God who works in you to will and to act according to his good purpose.In the previous section, Paul encouraged - even instructed - the Philippian community to be unified. His words: easy to understand, harder to do.
This section is a followup, or final push to encourage the Philippians to find unity among themselves. Notice how Paul attempts to persuade them to follow through - even if the task seems too difficult or uninteresting: (my paraphrase) "In the past, you followed my instruction while I was with you. Now, in my absence, it's all the more important for you to do so. And, if you think it's too hard - or you just don't want to - remember this: It is God who is working in you, giving you the desire to do and empowering you to do what He desires."
Paul's letter to the Colossians had a similar phrase: (1:29 - emphasis added) "To this end, I labor, struggling with all HIS energy, which so powerfully works in me."
So, once again, we can say that we work, and God gets the credit. Why? Because He works in us to give us the desire to work, and He gives us the energy to work. Why be motivated to work towards unity? Because that is God working in us!