​Philippians 2:12

  • ¶ 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. (Philippians 2:12)

This is a verse often used to say that our salvation is in our own hands, that it depends on our work.  Looking at the surrounding verses in the same chapter (and even the surrounding chapters) to gather context, I would say they instead support the notion that Paul was giving an exhortation to believers to do good works for the glory of God, not that this verse was a warning about how one can lose their salvation.

Starting in the previous chapter, you can see that the book of Philippians was addressed to saints (those who are saved), in disagreement with those who claim Philippians 2:12 tells people how to get saved:
  • ¶ Paul and Timotheus, the servants of Jesus Christ, to all the saints in Christ Jesus which are at Philippi, with the bishops and deacons: (Philippians 1:1)

These are a couple of the verses of exhortation I'm talking about:
  • [Let] nothing [be done] through strife or vainglory; but in lowliness of mind let each esteem other better than themselves. Look not every man on his own things, but every man also on the things of others. (Philippians 2:3-4)
  • ¶ Do all things without murmurings and disputings: That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world; (Philippians 2:14-15)

We also have the example given of Jesus Christ and how He acted:
  • Who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. (Philippians 2:6-8)

And yet more about the good works of the saints:
  • Yea, and if I be offered upon the sacrifice and service of your faith, I joy, and rejoice with you all. (Philippians 2:17)
  • Because for the work of Christ he was nigh unto death, not regarding his life, to supply your lack of service toward me. (Philippians 2:30)

Gathering that context together bolsters the claim that Paul is inciting us to do good works to glorify God.  As we work out our bodies to make them stronger, we can "work out" our heeding to the Holy Ghost like a muscle.  We can try to live more Biblically, and as that's basically what discipleship is, and discipleship is one of the hardest things to live under on this earth, it's understandable that it entails fear and trembling as we are weak:
  • Watch and pray, that ye enter not into temptation: the spirit indeed [is] willing, but the flesh [is] weak. (Matthew 26:41)
  • Servants, be obedient to them that are [your] masters according to the flesh, with fear and trembling, in singleness of your heart, as unto Christ; (Ephesians 6:5)
  • Serve the LORD with fear, and rejoice with trembling. (Psalm 2:11)
Another thought that comes to my mind is that, as there are consequences to our actions, we stand in fear of those consequences and can try to act wisely instead:
  • And ye have forgotten the exhortation which speaketh unto you as unto children, My son, despise not thou the chastening of the Lord, nor faint when thou art rebuked of him: For whom the Lord loveth he chasteneth, and scourgeth every son whom he receiveth. If ye endure chastening, God dealeth with you as with sons; for what son is he whom the father chasteneth not? But if ye be without chastisement, whereof all are partakers, then are ye bastards, and not sons. Furthermore we have had fathers of our flesh which corrected [us], and we gave [them] reverence: shall we not much rather be in subjection unto the Father of spirits, and live? (Hebrews 12:5-9)
And trembling also fits because at some point in the future I believe we'll have to answer to God for how we chose to serve Him after we were saved.  What the consequences of that are exactly, I'm not sure, but it has nothing to do with us leaving heaven.

Even presenting the facts about Jesus can be a fearful thing at times, as it was with Stephen, so this provides another possible answer:
  • ¶ When they heard these things, they were cut to the heart, and they gnashed on him with [their] teeth. But he, being full of the Holy Ghost, looked up stedfastly into heaven, and saw the glory of God, and Jesus standing on the right hand of God, And said, Behold, I see the heavens opened, and the Son of man standing on the right hand of God. Then they cried out with a loud voice, and stopped their ears, and ran upon him with one accord, And cast [him] out of the city, and stoned [him]: and the witnesses laid down their clothes at a young man's feet, whose name was Saul. And they stoned Stephen, calling upon [God], and saying, Lord Jesus, receive my spirit. And he kneeled down, and cried with a loud voice, Lord, lay not this sin to their charge. And when he had said this, he fell asleep. (Acts 7:54-60)