This is a very loosely held opinion, though I'm not willing to go back and forth on it, and I doubt my flag will change anything one way or another.
From an HN moderation point of view, religious tolerance follows from the value of intellectual curiosity, which is what we're optimizing for (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...). Not all religious posts are on topic, of course (most aren't), but they are when there's overlap with intellectual curiosity, the same as with other divisive themes (https://hn.algolia.com/?dateRange=all&page=0&prefix=false&so...), and when that's the case, tolerance is paramount.
Religious tolerance has been a prime principle of intellectual culture for many centuries. If you zoom out to a historical perspective, it's clear that it's in all our interests to practice it, regardless of what our religious or irreligious views may be. We don't have to hold the same views to respect each other, and the art of interesting discourse with people who hold different views is something we should all cultivate, assuming that we have the intellectual curiosity that the HN guidelines speak of.
From a different angle: the problem with mobile apps as Show HNs is that they're not easy to try out (https://news.ycombinator.com/showhn.html) without going through the rigamarole of installing them. But that's a problem with the entire category, and as long as we haven't disallowed them (which we're not going to), this submission seems as valid as any other.
I can adjust my behavior if this wasn't the right way to think about it, but I just want to make sure I'm acting as intended with this point of view.
I wouldn't say you were super wrong about how the system works or is intended to work! I'd say you and the other flaggers misapplied the site guidelines in this case. You're right that it's moderation's job to correct for such failure modes.
Please consider that there are varying perspectives on HN beyond your own and that sharing an app which allows people to study the Bible isn't forcing a worldview on anyone.
Even if I don't like the product, it's certainly worth a discussion at least.
The problem is, unless I'm misunderstanding something about HN, they can not vouch until the submission is already dead.
You're not out of bounds to suppose this will have some bad comments, but I do think it's presumptuous to bet there will be no interesting discussion. There in fact has already been both - a lot of interesting discussion, and a little bit of bad comments. I encourage you to flag bad comments while they are still the minority, and only resort to flagging the submission if they take over, since, again, nobody can vouch for the submission until it's dead, at which point there will be fewer people to vouch for it, as it will be hidden.
Edit: Actually, maybe vouch is only for comments, and not for submissions? I'm not sure exactly how it works, but I think my suggestion still makes sense.
On a side note, I'm curious the history of "lowest common denominator". Did it start out as a confusion of "least common multiple" and "greatest common divisor/denominator" from mathematics, or was it more purposeful?