Something had already happened in Philippi before 2:12

Philippians 2:12 So then, my beloved, just as you have always obeyed, not as in my presence only, but now much more in my absence, work out your salvation with fear and trembling; 13 for it is God who is at work in you, both to will and to work for [His] good pleasure.

If God hadn't worked in the Philippians before (2:13 says He is at work in them), Paul could be telling them to work out many things, but how could he be telling them to work out their salvation? "Your salvation," that is, "your own salvation" (RV et al), is  την εαυτων σωτηριαν.  Let's recognize the plurals, including  "your (pl) salvation."  "Ye have always obeyed," says the AV/RV/ASV/YLT, which keeps the "ye" form, reflecting the plural address.  So they were being told by Paul to work out their salvation.  All those other translations, plus the RSV/ESV, from our times, shows the emphatically placed εαυτων adjective as present, by translating "your own salvation."  Poetically sounding English, reflecting even more the Greek would say "the you-folks'-own salvation," the salvation that is you-folks' own!

What does this rule out?  Acquisition!  If their salvation is their own, it is not something they are to acquire.  This simple Greek word εαυτων here has a Protestantism built right into it, we could say! ... at least, a Protestantism as it had started, which Trent remonstrated against back in 1546, against which the natural man rails, saying, how dare God presume to give me that great sine qua non, which I cannot say I must do some one good thing to obtain (Mt 19:16), or even possibly many things, to inherit (Mk 10:17; Lk 18:18) and establish (Rm 10:3)? How dare Paul tell them that something, salvation no less, is already their own.