It's not talking about reverence?
When Paul sent Titus to the Corinthians, they received Titus with "fear and trembling."
Were they afraid of Titus? Were they trembling in fear of him?
They received him in "fear and trembling" knowing who sent him, Paul. They were showing humility and reverence to Paul for his position as an Apostle of Christ.
Back to "workout your salvation in fear and trembling" is exactly the same meaning.
The next verse tells us that it is God working in us, the Holy Spirit is God, that's who Paul is referring to.
So, "working out your salvation in fear and trembling" and "for it is God who works in you" is showing humility and reverence to God the Holy Spirit as He works in you. In other words, as I said before, cooperate with the Holy Spirit in humility and reverence.
If you were to read both 1 Corinthians and 2 Corinthians you would know that the Corinthians had sinned (1 Corinthians 3, 1 Corinthians 5). Paul explains in 2 Corinthians 7:6-15 explains how they sought forgiveness with the Lord Jesus with a godly sorrow, and they had by forsaken their sins (i.e. repentance - 2 Corinthians 7:10). So their receiving of Titus in fear and trembling was most likely in their wondering if God was going to forgive them or not by the coming of Titus.
I say this because Paul says, “Again, think ye that we excuse ourselves unto you? we speak before God in Christ: but
we do all things, dearly beloved, for your edifying. For I fear, lest, when I come, I shall not find you such as I would, and
that I shall be found unto you such as ye would not: lest
there be debates, envyings, wraths, strifes, backbitings, whisperings, swellings, tumults:
And lest, when I come again, my God will humble me among you, and
that I shall bewail many which have sinned already, and have not repented of the uncleanness and fornication and lasciviousness which they have committed.” (2 Corinthians 12:19-21).
Meaning, Paul worried about them because of their sin.
So it's highly probable that the Corinthians worried because of the sin they committed, too. They were awaiting to see what Titus was going to say (seeing he was an authority sent from the apostle Paul from God).
Paul also says,
“Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates?” (2 Corinthians 13:5).
Anyways, the problem I have with your belief Charlie is that you keep having to change or ignore the verses I put forth to you. If we were just talking about one verse like Philippians 2:12 and that was the only one in the Bible that appears to refute your false belief in that declare that you are sinner (present tense) saved by God's grace then there is a small percentage of a chance you could be correct. But the sheer number of verses on the warnings against how sin can destroy our soul, and or in being cast out because of unfruitfulness of not seeing God because of unholiness should give you pause to reconsider. But you simply don't like the many verses in the Bible that do give us these warnings. I have provided many of them to you so far, and you keep having to change them. That's what concerns me, Charlie. We are not talking about just one verse here; But many.