The person who believes ‘once-saved-always-saved’ must believe that the warning passages are counter-factual conditionals. I’ll explain those terms (in case you’re not familiar w/them). A counterfactual asserts that some statement is true, where that statement describes something that isn’t the case (or won’t be the case). For example, ‘if I wasn’t writing this comment, I would be doing something else.’ A conditional is an ‘if..then…’ statement. So a counter-factual conditional statement, is an if-then statement about would would happen if something else happened.I think this is a bit to simple. If a conditional is contrary to empirical fact that is very different than making a conditional based on an impossible premise.
The best way for the person who believes ‘once-saved-always-saved’ to interpret the warning passages is as a counter-factual conditional statement. In other words, the warnings would be interpreted as making a claim about what would happen if one apostatized. So they are true statements; if one apostatized, then x occurs. It’s just that they are counter-factual because the believer won’t apostatize (per the once-saved-always-saved view). It’s not that it’s somehow *impossible* for the believer to apostatize. It’s just that the true believer won’t do so.
If McCain had won the election then he would appoint pro-life judges.That is a counter-factual conditional based on a possible premise. It is always going to be true in the trivial sense because the if condition didn't happen. But there is a sense in which this could be false. McCain could say he would not have appointed pro-life judges. That would not make it false in the strict logical sense but the statement is trying to convey something beyond the strict logic. That something could be true or false. If it is false then calling the statement true is quite perverse.
If the holocaust was a hoax then the Jews have a lot of explaining to doThat is a counter-factual conditional based on an impossible premise. So it logically true. But it is offensive. Why? Because making such a statement seems to imply the possibility that the if clause might be true. It does not say it but it implies it.
So when a passage of scripture says "if one apostatized, then x occurs" the Calvinist cannot escape by just saying the statement is trivially true because the if clause is false. There is an implication that this case can happen. If that is not the case it should be made clear that this is a pure hypothetical. Like Paul talking about "if Christ was not raised ...". He makes clear that he does not think this possibility is real. If he did not it would be a very perverse way of speaking.
Now if you only claim scripture is inerrant you can use this tactic to get around these passages. But if you claim scripture is inspired or God-breathed you have a problem. Inerrant simply claims no errors. It can be unclear. It can be a deeply flawed way of expressing an idea. That is all consistent with inerrant.
Inspired means God is the author. It should not be perverse. It should not be misleading. It should not be confusing. That is a much stronger statement than inerrant. Counter-factual conditionals where the if clause is absolutely impossible should not appear in something with God as it's principle author. It would mean God is implying falsehood without technically speaking falsely. It means God speaks like Bill Clinton and hides behind what the word "is" could possibly mean.
Catholics can do this with infallible documents because they don't claim they are inspired. They are merely protected from error. They could contain misleading statements. They just cannot contain false statements. They cannot do it with scripture. Scripture is something we need to be constantly reading and contemplating. It does not obfuscate simple truths. It gives insight into great mysteries.