Philippians 2:12-13 is one of Paul's most famous statements and it's one we forget to place in the context of the letter. At first reading, it looks like Paul is highlighting a paradox of YOU work out your salvation because GOD is working in you. But in context, I think Paul is still concerned that the Philippians are living their lives in a manner worthy of the gospel (1:27) and are motivated to do so for the right reasons.