Verse of the Day Philippians 2: 12-13

"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, for it is God who works in you, both to will and to work for i his good pleasure."

This is where I always get confused
I mean Paul is speaking to believers in the faith, so when he says work out your salvation he is not talking about the regeneration, meaning he is not saying you could work to gain salvation
He is addressing those who have always obeyed, to keep obeying, not content with the glories of their past obedience but to continue to learn and act as Christ followers - this is why Paul follows the verse with - do all things without grumbling or questioning - he is continuing to show the Philippians how to act
The amazing thing is that it isn't us in that effect either "for it is God who works in you"
God is the one bringing salvation and He is the one who is working out this salvation in us
Our 'job' or call is to obey Christ with fear in trembling, not worrying about the final outcome for we know that God is working out for His good pleasure
Where I get confused is when Paul says that he ma be proud that he did not run in vain,
so how could Paul's efforts and work for Christ in teaching the Philippians and other church be vain if God is the one who works out our salvation to His good pleasure
God's efforts are never thwarted and His glory is never diminished, so does that mean none of our efforts for His kingdom are in vain, and if that is true what is Paul saying?
Well I'm not entirely sure yet so I'm going to ask and look it up
But my thought on it is that although we are called to live an abundant life in Christ and God works in us, we turn from that and sin Paul wants to make sure that from his efforts that those he is leading would be living in Christ's abundance and proclaiming Him as well