Philippians 2:12 is one of the most commonly misunderstood and misinterpreted passages in the whole Bible. Many Catholic Apologists and other "Christian" reprobates quote this passage as proof a Christian must work for their salvation. Let's look at the verse and read it exactly as it says without reinterpreting it.
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.
All you need is an understanding of english to understand this verse.
The verse says "work out your salvation". It does NOT say "work for your salvation". Let's use an everyday example of this type of phrasing just to give you an example of what working out something means. If I were to say to someone that goes to the gym "work out your muscles" it clearly implies the man already has muscles and he is merely strengthening them. However, if I were to say "work for your muscles" it would clearly imply the man has a lack of muscles and I am encouraging him to get some muscles by working out.
The exact same thing applies to this verse. Paul was warning the Philippians not to be a lukewarm Christian. So he tells them not to just be content with their salvation, but to work it out with fear and trembling.
This verse is used by heretics to claim Paul taught works salvation. This only shows how void they are of understanding God's word.
1 Corinthians 2:14
But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.