Cited from pureunadulteratedgrace blog: The words "fear and trembling" can mean your knees knocking with fear. However, Paul in most cases used it to mean "reverence and awe." I believe Paul was implying reverence and awe here, but it has more to do here with distrust in one's own abilities. Strong's Concordance states that a distrust in your own abilities can also be meant:
2) with fear and trembling, used to describe the anxiety of
one who distrusts his ability completely to meet all requirements, but religiously does his utmost to fulfil his duty
We can see this distrust of one's own abilities used in 1st Cor. 2:3-5:
[SUP] 3[/SUP]And I was with you in weakness, and
in fear, and in much trembling.
[SUP]4[/SUP]And my speech and
my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power:
[SUP]5[/SUP]That your faith should not stand in the wisdom of men, but in the power of God.
Here we can see that Paul used such words to describe distrust in his own abilities. He did not come with "enticing words of man's wisdom" (distrust in his own abilities), but in the power of God.
I find the message here to be similar to Philippians 2:12-13. Paul was telling the Philippians to continue working out their salvation with a distrust in their own abilities.
Paul told them to not to trust their own abilities as verse 13 gives the reason why:
[SUP]13[/SUP]
For it is God which worketh in you both to will and to do of his good pleasure.
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Pure Unadulterated Grace: Philippians 2:12-13