I don’t think I understand this question too much. Are you asking if, commuting sin as a non believer, it’s still sin, or if people who are believers would still act as they do, if the idea of gods and religions didn’t exist? For the latter, my guess is that religion and gods are born of the ideas of men, so we’d probably still act as we do now, only the behaviors, justifications and beliefs would be attributed to whatever system we’d have in place of known religions. Because I think it’s highly unlikely, given human evolution and societal structuring that we could ever NOT have some systems.
But is a sin still a sin of the person doesn’t know it’s a sin? Well, according to The Bible, not believing is a sin, isn’t it? (correction appreciated) Might depend on the religion and its belief. It can pretty much go two ways only.
Say vampires exist, and say I believe in God, and I love Him. I’ll use a cross against the vampire, and my faith will protect me against the vampire. But an atheist using the cross won’t be blessed by God, so the cross won’t do anything.
OR; just having a tool that represents the power of God is good enough no matter who wields it. It works by default, no matter what your beliefs are because the tool is imbued with the power of God. So either you sin and you know it’s a sin, or you sin and you don’t KNOW it’s a sin, or, seeing as you don’t know, your ignorance protects you until you learn, or die…in which case, I wonder what happens then…limbo? Purgatory?
TLDR; with or without religion, I don’t think mankind’s behaviors would change much at all. that is, if I understood anything about this question