Philippians 2:
9. Wherefore God also hath highly exalted him, and given him a name which is above every name:
10. That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth;
11. And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father.
Paul will now tie that to the reason why [here, the word wherefore = for this reason] he is asking the following conduct from the Philippians:
12. Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
13. For it is God which worketh in you both to will and to do of his good pleasure.
14. Do all things without murmurings and disputings:
15. That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world;
16. Holding forth the word of life; that I may rejoice in the day of Christ, that I have not run in vain, neither laboured in vain.
When a Boxing coach instructs a Boxer frustrated in some area of his skill to "go hit the heavy back a few rounds; work out your frustration" what is being implied as to where said frustration is?
Within that Boxer. He is not working for his frustration, rather; he is working it out; he is letting it out of himself and into that heavy bag.
Same thing - when the Believer first trusts Christ, the Spirit of God saves Him. And He places His salvation in Him the moment He Himself comes to live in him.
The Believer is given the Spirit of God. But things don't end there.
Through God's Word residing in the Believer as the Believer invests time in His Word learning about the details of who God has made Him in His son, and then walks by faith in said Word, God empowers the Believer through His Word by the Spirit, both to will and do of His good pleasure.
When the Believer walks in reverence to this, he is said to be working out, manifesting, or walking in this salvation that God has placed in Him by His Spirit, the moment in which He first trusted Christ.
The Philippians had been able to work out their salvation toward one another when Paul had been around, out of reverence to him; the way people tend to behave toward one another around "the Pastor."
Paul is asking them to work out their salvation toward one another; to manifest it toward one another, in his absence also.
Paul is relating that the key to being able to do that, is to revere that this is God's desired pleasure. It is God's desired pleasure that the Believer allow God to be the will behind his actions.
Because it is God's desire that the Believer "be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world."
How? By Holding forth the word of life," as the Believer ought to, because he is a son of God.
Their persecutions were impacting how they were behaving toward one another.
Thus, Paul's earlier words along this same line of thought on his part as to his desire in the Lord, for them, in chapter 1:
26. That your rejoicing may be more abundant in Jesus Christ for me by my coming to you again.
27. Only let your conversation be as it becometh the gospel of Christ: that whether I come and see you, or else be absent, I may hear of your affairs, that ye stand fast in one spirit, with one mind
striving together for the faith of the gospel;
28. And in nothing terrified by your adversaries: which is to them an evident token of perdition, but to you of salvation, and that of God.
The world might view their persecutions as an evident sign that God is a myth, but to you Grace Believers, asserted Christ through the Apostle Paul, it is to be an evident sign that the persecutions are only because the Father has delayed His vengeance on them that know not God, in that day that every knee shall bow; God has delayed that day one more day, in His kindness and His grace toward this world in His Son, that He might save some.
Wherefore - for this reason - work out your own salvation before this dying world - let the world see this Grace God and our Savior in His Son by the Spirit working in your fellowship with one another, both to will and to do of His good pleasure.