A recent Patheos essay by John Beckett worried that conversations about piety and ritual impurity are bringing the concept of "Sin" into Paganism. For him this would be an unqualified Bad Thing since:
Sin is breaking the rules – even if those rules are arbitrary and outdated. Sin is transgression – even if the institution we transgress against is regressive and harmful. Sin is error and “missing the mark” – even if that mark is impossible to attain. The concept of sin tries to force a rainbow world into a black and white box.
Avoiding sin requires perfection, and since perfection is unattainable, we’re told we’re bad and evil. We feel shame for shortcomings we could not possibly avoid, some of which aren’t even shortcomings. Christianity’s answer is that a god-man will vicariously impart perfection to believers. The proposition works for some, although by their own admission they never completely stop sinning.
A straw man nailed to a cross is a wonderful prop if you're staging Children of the Corn. It's less impressive as a debate tool. There is no shortage of Christian literature on Sin, and I wonder why Beckett didn't consult Fr. Google to see what actual adherents had to say on the topic. Since I am a cultural Roman Catholic, I'll begin my search with the New Advent Catholic Encyclopedia. There is of course a good bit of RC-specific material here. But there's also a fair bit which could prove useful to people of any religious persuasion. For example:
Sin is nothing else than a morally bad act (St. Thomas, "De malo", 7:3), an act not in accord with reason informed by the Divine law. God has endowed us with reason and free-will, and a sense of responsibility; He has made us subject to His law, which is known to us by the dictates of conscience, and our acts must conform with these dictates, otherwise we sin (Romans 14:23).
The pre-Christian world was quite capable of creating moral codes and of understanding these moral codes would be violated. Centuries before Moses purportedly received the Ten Commandments Hammurabi's code was followed throughout Babylon. The codes of their communities certainly included proscriptions which appear silly to us, just as many of our "thou shalts" and "thou shalt nots" would seem odd and arbitrary to them. But they would be puzzled indeed by the idea that "avoiding sin requires perfection" when it simply meant "behave in accordance with community standards." And the pre-Christian world certainly recognized "Divine law," though their take on said law admittedly differed from the Catholic vision.
New Advent goes on to note that Sin must involve a conscious decision. The sinner is aware the action is wrong yet for whatever reason chooses to violate the moral imperative. This is useful in distinguishing between sin and ritual impurity. You can be contaminated by willful wrong action but you can also be contaminated by proximity or contact: miasma involves no moral payload. It's also useful in stopping the kind of free-floating anxiety Beckett describes. If you are honoring the Gods to the best of your ability with the knowledge you have at hand you are not going to fall into a state of Impietas. Christians may think everybody can do it -- but sinning really takes an active effort!
We can even consult other experts on the subject. In The Satanic Bible Anton LaVey repeatedly praised the Seven Deadly Sins and noted they led to self-gratification. But the Church of Satan is really more about naughtiness than wickedness. Satanists enjoy tweaking the bluenoses (and their founder wrote about the "Law of the Forbidden"), but their parties owe more to Epicurus than Hannibal Lecter. LaVey distinguished between Indulgence and Compulsion. Throughout his career he condemned drug abuse and "druggies" and disparaged efforts to treat substance abuse or criminal misbehavior as a sickness rather than a personal failing. So while Satanists may make light of the John Beckett version of "Sin," they hardly reject the idea outright. Current CoS High Priest Peter Gilmore has described their worldview as "I-theism." A religion of self-deification certainly can (and does) condemn acts of wrongdoing which cheapen the wrongdoer before the community and hirself.
The idea that "all have sinned and fallen short of the glory of God" (Romans 3:23) is Christian: in the pre-Christian world it was expected that most would uphold community standards and honor the Gods, and violations were the exception rather than the rule. And while castes and familial curses certainly exist outside Monotheism, the idea that we are all tainted by some sort of "original Sin" is hardly universal. That being said, it's ahistorical in the extreme to imagine "do whatever you want so long as nobody gets hurt" was the order of things before the Monotheists came in and spoiled the fun. They may not have split the world into "Good" and "Evil" but they certainly knew the difference between right and wrong.