Phil. 2:12-13

12 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, 13 for it is God who works in you to will and to act according to his good purpose.

Remember, Paul was writing to the people who made up this church; the Christ-followers in Philippi.  He’s writing to people who are already Christians, so “work out your salvation” doesn’t have anything to do with how someone becomes a Christian.  In fact, salvation is through faith in Christ alone and has nothing to do with any works we might do (Titus 3:3-8, Eph. 2:8-9).  

Paul is writing to encourage them to live out their salvation with the constant reminder to live in awe and reverence of God.  This fear shouldn’t cause us to live in anxiety about God, but is the kind of “fear and trembling” that causes us to realize the awesome responsibility we have to represent Him by the way we live.  The ultimate penalty for our sin was paid for by Jesus on the cross, but there are still consequences for our sin here on earth.  That’s why we need to confess our sin regularly (1 John 1:9).

The new testament is clear that for every genuine Believer – Christ-follower, there was a point in the past where that person “was” saved  (born again).   As this Believer lives his/her life here on earth in the present, they are “being” saved (sanctified) and are to “work out” their salvation as they live.  They are utterly and eternally saved when they genuinely put their faith in Christ so this has nothing to do with “working for” their salvation, it’s just the process of growing in that salvation while on earth.  When this Believer is finally with Jesus in the future, they “will be” ultimately saved and receive a glorified body just like His.  

The way cool thing in the midst of all of this is to remember that God is at work in us to accomplish his work and purpose, and He has promised to complete what He began in us (Phil. 1:6).  That’s good news that ought to bring us great joy!