From what I imagine as the agnostic position, the “sin” is the perpetual self-exclusion from membership in greater groups that provide meaning for your life. While the metaphysical is-and-always-will-be completely unproveable in any empirical sense, that doesn’t necessarily mean that religious ideas lack some unambiguous merit.
For my best example, I get my concept of “right and wrong” from my faith, as we can probably agree that those ideas are purely metaphysical conjecture. The closest we get to those ideas in other species is “normative behavior” like we see with dolphins and apes. As a result, my concept of “right and wrong” mashes reasonably well with those of Abrahamic persuasion as well. Thus I can start to establish a sense of “tribe” with those people. And in order to further strengthen my desire for tribal membership, I concede to Western Christianity (specifically Catholicism) as that religious view is clearly the source of the moral code for the majority of the western society in which I live.
To be sure - when I see an atheist like Penn Jillett (sp?) object to religious morality with a quip such as “I DON’T NEED GOD TO KNOW GENOCIDE IS WRONG”, I am certain that his belief is simply the residue of the western society he grew up in as well (which was founded with Christianity as it’s moral source). If you were to press him on “why” genocide is wrong, his answer would necessitate an appeal to some sort of abstraction similar to the concept of “god” - which is also subject to the same critiques that Penn uses to debunk “god”.
Sure, he might spout something about the “golden rule”, but upon the lightest of analysis, the “GR” is merely a call to consistency. If you think poor people deserve to starve, then when you starve you must refuse offerings of food
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And as long as you’ll abide by that, the GR can be safely used to let poor people starve to death.
Really, the only morality that nature unambiguously gives us is “right of might”. As a tiger, if I’m hungry and I’m capable of hunting and killing you, then you should be my next meal. And you should be “ok” with that. After all, if a tiger can’t kill, it can’t eat and will die.
In sum, consider Christianity because it’s the source for the moral fabric of the world you live in - if you’re a westerner. If you lived in Japan, the rationalist in me would strongly suggest Shinto. In that way, a Japanese man can be more fully Japanese and a westerner can be more fully western and thus derive more meaning and satisfaction from their lives as those beliefs allow them to more fully interact with the fellow members of their society.
As to the positives of agnosticism? I think I provided in the top of my post.
And a note to the hardcore Catholics who will blast me for this post - I’m trying to meet him where he is. Not where I am. Chill out.