When Paul told his spiritual children that made up the Philippian church to continue to obey (Phil 2:12), with the added charge to “work out your own salvation with fear and trembling…”, he was speaking from a plane of spiritual understanding most of us don’t operate in.
I cannot pretend to understand fully what this charge means — at least not yet. I do know some of what it means though, and even more of what it does not mean.
Working out salvation is not working “for” salvation. Like going to the gym, growing more of the muscle that God gives me is my responsibility. However, I cannot create the muscle, and I did not give it to myself to begin with. I can only work out what God has already given me (and this is not at all a matter of discipline).
Yet to even compare this charge to a working out of a physical sort is to begin to derail from the point Paul was making. The word that is translated as “work out” is the greek word “katergazomai”, which is to literally accomplish and bring about to an exact finality. So the analogical concept of a muscle will obviously not work here beyond simply helping us understand that we didn’t give it to ourselves. Paul is not saying to “continually try to make it bigger, stronger, and more powerful in spite of resistance”. He is saying, literally, to accomplish it completely.
This is where the plane of Paul’s understanding is seemingly beyond mine, so far at the age of 29 years old, because I can so hardly explain how this works that I would never think to tell someone to do this as though it were somehow their responsibility. I can know that it does work though and it is our individual responsibility to “obey”
What I see from scripture and know to be true from experience is that the Spirit of God, deposited within me when I placed my faith in Jesus (Eph 1:13), is the one who continually motivates me to walk in obedience to what I know to be true each day. This is why Paul finishes his charge with the second half:
…for it is God who works in you, both to will and to work for his good pleasure. (Phil 2:13)
Furthermore, as Paul already said in this letter to the Philippians, the Spirit of God within me is the one whom will bring me to completion on the day I am with Christ.
And I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ. (Phil 1:6)
Therefore, if I possess salvation because of God giving me his Spirit, I will no doubt continually choose obedience because of this saving Spirit — not for the sake of it — until this Spirit is fully and completely what I live in accordance to.
This will be complete when we shed this corrupted flesh and we stand face to face with Jesus in a new heaven and new earth in a new flesh in full unity with God’s Spirit.