"Really? What's so original about it?" ~Me
The doctrine of original sin says, basically, that all human beings are born sinful. Depending on the emphasis, we may be "born sinners" or "born with a sin(ful) nature" and therefore destined, inevitably, to sin. Either way, this is said to be a result of The Fall, which occurred when Adam and Eve disobeyed God and ate from the Tree of Knowledge of Good and Evil. (Interestingly, there's a line in Genesis which suggests that God was not just punishing their disobedience; He was afraid they would also eat the fruit of the Tree of Life, and live forever, and become like Him.) So, basically, Adam and Eve broke the rules, and the rest of us have been paying for it ever since... but I digress.
This is one of the - I guess I'd call them "foundational beliefs" - of mainstream Christianity. That said, I'm not sure it's strictly necessary - it's not part of the Nicene Creed, for example - and I imagine that one could still be a Christian without holding to this particular view of human nature. But among the people who do believe it, I most often hear it used as an explanation for why everyone needs the redeeming love of Jesus Christ. In the more negative approach, it's the justification for why it's okay that everyone who doesn't accept Jesus will end up in Hell.
Obviously, I don't believe this myself. I don't for a moment accept that babies are born as sinners; I'm not even sure what the term would mean when applied to someone who can barely distinguish between the Self and the World. Similarly, I just can't believe that people do horrible things to one another only because our two most distant ancestors once ate a magic fruit.
But people do accept this story. On some level, it resonates. I think that's because there's some truth to it.
Here's the way it usually goes: person 1 points out that we are all born sinners. Person 2 starts to object. Person 1 says, more or less, "Think about it. Nobody goes through life without sinning." And person 2 usually finds themselves nodding their head and saying, "Well, I guess that's true."
And it is true. Nobody goes through life without making mistakes, doing things they regret, or hurting other people - not if they survive to adulthood, at least. But that's not the same thing as saying that we're born with a sinful nature. The two statements are juxtaposed as though the second supported the first... but it doesn't.
In truth, I do think it's impossible to go through life without sinning - "sin" being defined, in this context, as "doing things you shouldn't". This is not because we're born sinful, but because we're born ignorant. Morality is something we learn; and as with everything else, we learn it largely by trial and error. You can't learn that way without, well, errors. We refine our morality by seeing the effects that our actions have on others - and sometimes by observing the effects of other people's actions as well.
To come at that from a different direction, I tend to see "sin" or "evil" as a quality which describes certain actions, rather than a character trait. So, saying that we all do some evil in our lives makes sense to me. Saying that we're all sinners, on the other hand, is no more meaningful than saying that we're all saints: neither assertion is entirely false, but neither is a complete or even particularly helpful description of the human condition.
So, yes, everybody does things they shouldn't, and sooner or later everybody needs forgiveness, if only from themselves. But there's a big difference between "I've done bad things" and "I'm a bad person." (That's not to say that there are no bad people; if someone makes a habit of doing bad things, then I feel fairly justified in calling them a bad person. But, again, it's a matter of actions - of patterns of behavior - rather than intrinsic nature. They're a bad person because they do bad things; it's not that they do bad things because they're a bad person.) And when it comes to the assertion that we are all, by nature, bad people - I just don't buy it. We are all imperfect people, but that is not the same thing. The difference may be subtle, but it's important.
This didn't come together as well as I would have liked, and you're welcome to poke holes in it if you disagree. I may take another run at it later on. We'll see how I feel about it later.