Philippians 2:12-13
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.
Philippians 2:12 and 13 go together; you can't understand verse 12 apart from verse 13. This is made clear by Paul's use of "for" at the beginning of verse 13. This term signals that verse 12 is conditioned upon verse 13. In other words, verse 12 is possible only because of verse 13; if verse 13 isn't true, then verse 12 isn't possible; if God hasn't first worked into us the ability and desire to do His will, we can't work out our salvation with fear and trembling.
So, then, verse 12 isn't indicating that we should be afraid for our salvation but that, in a condition of deep, awe-filled reverence for God and Christ before whom every knee will one day bend and every tongue confess that he is Lord (verses 10-11), we work out what God has worked into us by His Spirit. Nothing rests upon me except to manifest in my living what GOD has first done in me. I certainly can't keep my salvation by my works. I didn't obtain my salvation by this means (Eph. 2:8-9; 2 Ti. 1:9; Tit. 3:5) and my acceptance by God rests solely upon my being "in Christ" (Ro. 13:14; Ga. 3:27; 2 Co. 5:17; Eph. 1:1-13) so what, then, is there to fear concerning my salvation? It is as secure as God's eternal, unchanging acceptance of Christ in whom I have obtained perfect redemption, justification and sanctification (1 Co. 1:30).
And so, when Paul wrote what he did in verse 12, he was not suggesting that a saved person should be afraid for their salvation because it could be lost. Not at all. He was, in fact, indicating just how secure a believer's salvation is since it is conditioned upon the work of God in them who gives to His children both the desire and ability to do His will.