I don't want to argue with you, but I am curious. If they were already saved, why would it be necessary for them to work out their salvation with fear and trembling?
All the letters were written to the SAVED ( believers) There were letters of doctrine and/or instruction on how to live out your Christian life.
Read the people that Paul addressed it to
Phil1:1To all the saints in Christ Jesus who are at Philippi, with the overseers and servants:
1:12 Now I desire to have you know, brothers, that the things which happened to me have turned out rather to the progress of the Good News;
It was written as an instruction on how to live to be seen by the world. Notice the opening of that passage ... it is to THE BELOVED (the saved)
2:12 So then, my beloved, even as you have always obeyed, not only in my presence, but now much more in my absence, work out your own salvation with fear and trembling. 2:13 For it is God who works in you both to will and to work, for his good pleasure.
He then gives examples on how to do that and WHY
2:14 Do all things without murmurings and disputes, 2:15 that you may become blameless and harmless, children of God without blemish in the midst of a crooked and perverse generation, among whom you are seen as lights in the world, 2:16 holding up the word of life; that I may have something to boast in the day of Christ, that I didn’t run in vain nor labor in vain.
We all work out our salvation as we live it out . NO WHERE does Paul say that they are to do this to be saved, but like the letter from James it is a call to live your life so the unsaved can see your faith at work.
Eph 2:10 For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.