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, (Philippians 2:12, ESV)Paul is encouraging them. He is drawing attention to their previous obedience: He has seen their obedience first hand. He has experienced their loving care, as they, in obedience to God, had provided for Paul's needs, on more than one occasion.
So, what he says next is not because of a lack of obedience, but it is an encouragement to "keep on." Very much how one would see a runner, near the end of their race, a runner who had been running and was showing no signs of stopping... one might still encourage them as they were passing by, "Keep at it! Keep going! Don't stop!" Or how a coach might actually say, "Pick up the pace!" These words do not discount the previous effort, but acknowledge that effort and encourage it to continue.
And what is his encouragement? It is to "... work out your own salvation with fear and trembling..." It's yours! You have been saved! Live like it. Have that salvation that is in you, work it's way out into the world around you.
The "your" is plural in the Greek. This means that Paul has a church-wide "working out" in mind. He is seeing this church in Phillipi bringing their salvation into their work place, into their homes, into their social circles. Your salvation ought to be impacting your friends and your enemies.
One final thought, this verse, along with the next verse, are in my opinion, one of the most important passages in the scriptures, related to our understanding of man's responsibility and God's sovereignty. As we will see in the next passage, the motivation to work out our salvation comes from the sovereignty of God at work in that salvation.