Would it be moral or rational to try and save them from their own folly, or to write it up to personal choice instead?

We don't need to hypothesize fantasy land; this is a real life question. People very often make bad decisions, and aren't always warm to the idea of people telling them how to run their lives. It's their choice, but they'll still suffer from it.

Taboo the words 'moral' and 'rational'. To the extent that you value preventing suffering, you have to want to change their behavior. There may be conflicting factors, or orthogonal factors that dominate, but that term is still there.

I think a lot of unstated assumptions are being smuggled in with the word "god" (and related words like "worship"), resulting in confusion.

If I imagine I live in a world (W) where a powerful entity (E) punishes behavior it describes as immoral (B), and some people (P) engage in B anyway, and you ask me whether I ought to do what I can to prevent P from engaging in B or ought to write it up to personal choice... well, it depends on specifics.

E.g., if W = mid-20th-century Germany and E = the government and B = homosexuality, my answer is "neither." I ought to do what I can to protect P from E, and to get rid of E.

I assume that isn't what you mean by a "god I worship."

If I instead imagine I live in W2 where E2 is an extremely reliable source of moral judgments and judges B2 immoral, and P2 engage in B2, what should I do?

Well, the simplest answer is "whatever E2 judges." That is, I ought to ask E2 "Should I try to save P2 from their own folly, or write it up to personal choice?" and then do whatever E2 says.

What if I can't do that? Well, why can't I?

One possible reason is I don't know enough about W2 to know I ought to ask.

In the real world this seems plausible to me: I'm willing to believe that there's a system out there that reliably makes more accurate moral judgments than I do, but I don't know of any such system. But your use of the word "worship" blocks that reading... if I worship E2 in W2, it follows that I do know E2 has that property, because why would I worship it otherwise?

Another possible reason is because E2 doesn't answer such questions.

In this case, it seems to me that my next best bet is to reason by analogy from the things E2 has said, in order to answer the question. Which is more or less what believers in the moral superiority of various texts in the real world do. (Not just theists... a Objectivist friend of mine worked this way as well: when I asked him questions about what someone ought to do in this situation or that, his instinct was to go look the answer up.)

I realize that all of this sounds like I'm dodging the question, but to some extent that is my point: if you invite me to imagine that I live in a world that really does have a reliable source of objective moral judgments, and then ask me whether it's more moral in that world to do A or B, the correct answer really is that I should look it up!

The fact that such an answer doesn't tell me much that's useful about the real world is, to my mind, evidence supporting the idea that I don't live in such a world.

Would it be moral or rational to try and save them from their own folly, or to write it up to personal choice instead?

I'd say yes, within limits. For example if it was a fact that masturbation made you go to hell, I'd be in favour of heavily censoring porn, and finding ways to reduce the incidence of masturbation. But not necessarily chaining people's hands to their beds against their will (though even that is questionable - I would certainly be in favor of offering people the possibility of signing up for having their hands forcibly chained to their bed!).

By the way, you get roughly the same issue with drug use: is it moral or rational to try to save someone from becoming addicted to a drug that will ruin his life? (This is separate from the question of which substances can ruin someone's life)

Obligatory Ted Chiang: Hell is the Absence of God.