Sin is mysterious, arbitrary and criminally underexplained for how incredibly, vitally important it is.
Most people generally want to follow the rules. In a recent study in Nature, 55% to 70% of all humans, even when no one is around and no one is watching, will follow arbitrary non-zero-cost rules. Rules allow functioning societies to scale up via the establishment of expected behaviors sans explicit context and awareness (thus letting society grow beyond the bounds of any one human's knowledge), and thus exist for reasons.
Rules existing for reasons, and not for no reason, should be relatively uncontroversial. Any rule, law, moral prescript or expectation has to exist to serve a cause. A "Duty" without a reason for existing is senseless and arbitrary.
So let's talk about a very specific duty that has almost no explanation, despite being considered the reason we're being told we need salvation.
Why does sin exist? Not, "Why did Adam sin". Why does the system as a whole and in its entirety work this way? Why did God construct the framework known as Sin, categorize some behaviors as specifically "behaviors that are anti-God", and then make it so that humans, against their free will, are forced to inherit and incorporate it into their very base nature, and then need to believe that someone else's story is true in order to escape the consequences God designed for people to inflict on themselves?
A good moral framework has a reason for existing. Sin doesn't seem to. It's just arbitrary nonsense that God decided would be caused by humans disobeying him, after God decided to make the concept of obeying vs. disobeying for some reason, and then decided to make disobeying cause suffering for billions of people for some reason.
A good moral framework has clear rules. The concept of sin doesn't. What counts as a sin? What doesn't? Is it even a binary state, or is sin a sliding scale? The Bible never says. We're told some examples, but are never given any functional framework for sin identification - and as a result, humanity cannot even agree as what counts as a sin or not. There's no helpful indicators or useful tooltips in life indicating what to avoid, so even a good-faith attempt to be sinless may fail due to God's choice to not enumerate all sins or provide a proper heuristic for what is and isn't a sin.
A good moral framework relates the consequences of actions to the actions taken in ways that logically flow causally. Sin doesn't - it causes "separation from God", whatever that means, and might send you to hell, or might not, maybe sin doesn't actually have consequences and universalism is true, or maybe sin has consequences and some people burn for eternity, or maybe hell's temporary - no one knows, because nothing is explained and nothing has reasons and nothing follows rules and it's just chaos instead of a well-ordered framework on how to best live your life.
And I don't know about the rest of you, but I'm part of the group that just wants to follow rules and not cause harm. Why does the Bible have to make figuring out what counts as a sin and what doesn't so darn difficult? Why does it have to do absolutely everything to force everyone to have to be a sinner against their will? It makes no sense.
And people have tried to give me explanations - just had a good conversation with someone where they insisted they were choosing sin. Well, I'm not - I'm apparently having it forced on me. People told me to look at Catholic documentation, but when your explanation explicitly says, quote, "the transmission of original sin is a mystery that we cannot fully understand", that's not helpful. Generally, every discussion I've ever seen, instead of directly tackling any of the many questions I raised above, dances around them and tries to find satisfying and fruitful platitudes that make people worry less about the problem, rather than answering the problem. I state this as an explicit prediction of behavior for later checking, and dearly hope I'm proven wrong and people explain why the concept of forbiddenness had to and has to be a thing.