I believe the passage clearly indicates that Paul (months after visiting Corinth) was still sure that those who were "holding fast" were being saved. And furthermore, why. Because those who believed Christ had risen did not believe a vain Gospel. They believed a powerful/purposeful Gospel. One anchored squarely upon Jesus being the Christ, The risen God-Man
the gospel which I proclaimed to you, which you have also received, in which you also stand, by which you are also being saved,
No, it doesn't 'mean' that. In theroy, sure. But I note Paul doesn't say that some had 'let go'. He says some had believed (past tense, single event) in a un-risen Christ. Don't get me wrong, I do understand how this passage, in theory, could lead someone to theorize that people could 'let go' of a true salvific belief. But the technical fact is, the passage doesn't say anything about 'letting go'. Evidently Paul did too so he qualified his statement with "unless you believed in vain". Which is why that qualification is often left out by anti-OSAS posters.
Hopefully (that's my goal anyway) it means to me the same thing it meant to Paul.
The Greek word is really based upon a nautical term. As in the anchor is "holding fast". If you've done much boating, you know how important it is that the anchor is "set firm". If it slips hold because it really wasn't set firmly to begin with during the night or when someone is not paying attention, you are in trouble.
[I ONCE made a SCUBA dive with just me and a buddy on the boat. Came up 20 minutes later and you guessed it, the Boat was gone and us 10 miles out in the Gulf of Mexico. It took me 2 hours to catch up to the boat as it drug anchor in the wind. Pretty lucky really to have caught it. My buddy gave up swimming and let me go get the boat. I had to go back and get him.]
The point is though, the anchor was never "holding fast", else the boat would have been there when I surfaced. Paul knew the in's/out's of boat anchoring.