(This title makes me giggle. See? Calvinists laugh.)
Original sin is the Christian doctrine that people are prone to sin and cannot, by their own effort, be perfectly good. We are stained from our origins, from the very beginning of humanity. This is described in the story of Adam and Eve, who disobeyed God in the garden of Eden. The picture drawn in scripture is that their sin - the "original sin" - became a corruption that is passed down in the nature of every person. Some Christians read this metaphorically, and some read it literally, but every Christian church teaches that sinfulness is part of our nature before we are even able to make choices.
The Reformed church is not unique in teaching original sin, but it has insisted that original sin is a force alive in us, rather than something canceled at baptism. Our sin is forgiven in Christ, but it remains a force we must resist in this life. The Reformed church insists with a devout skepticism that we cannot, in this life, achieve perfection. We will not get better and better until we do everything right. Hard work will not make everything good.
Instead, the Congregationalist novelist Marilynne Robinson calls original sin "our inability to refrain from doing harm." The Anglican author Francis Spufford, whose book Unapologetic I highly recommend, abbreviates it as "the human propensity to fuck things up (HPtFTU)". Harm is entangled in our best actions. We do not and can not invent a sure method for separating out our admirable actions from our selfish ones. The flaw is alive in everyone.
So why on earth does this speak to my soul? Original sin (and the consequent "total depravity" - more on that later) is one of the most ridiculed of Reformed beliefs, often portrayed as a kind of theological meanness, a neurosis of the soul. This caricature seems to depend on the notion that sinfulness = worthlessness. When the Reformed church calls people sinful, the accusers claim, we are calling them worthless.
My father always taught me that the only fair way to criticize a belief system was in response to its best adherents. If you want to intellectually engage Hinduism, for example, you look at Gandhi, not a murderous mob in Orissa. The stereotype of the harsh, disapproving Calvinist who despises his fellow man might exist somewhere in reality, human nature being what it is (see what I did there?), but villains of that sort do not inspire Reformation. When we look at the best of the Reformed church, expressed in its historic confessions, we find that the doctrine of original sin seeks to confirm Christians three things: humility, mercy and giving glory to God.
The doctrine of original sin demands that I be humble about my achievements. I have not filtered out all the bad in my soul. It is still there, sneakily influencing me, and claiming otherwise is pretense. I am dependent upon rescue by a loving savior at every stage of life. In practical terms, I should be open to the correction of wise and holy people who see things I don't. The possibility that I am wrong or doing wrong should be lively in my mind. Until the world-to-come, I do not reach a point of settled perfection. I still screw up.
Knowing this - not merely my harm, but the inevitability of some harm - also demands that I show mercy to others who harm me. Cherishing slights and offenses is forbidden. I cannot demand of others a level of perfection that I know I do not possess myself. This is invaluable in congregational life. The bumps and bruises of life together can abound, and like a kindergarten teacher, this doctrine prods us all to forbear and forgive. The ordinary bumbling hurts of life tempt us to a shocked cry of "How could you?!" Original sin suggests instead a gentle, disappointed laugh, and an "Oh crap. Here we are again." This does not mean that evil goes unresisted, but it does mean that we frankly recognize perfection will not happen in this world.
Lastly, original sin acknowledges our inability to save ourselves. God has responded to this helplessness with the gift of Jesus, whose death and resurrection offer forgiveness for sin to all who receive it with faith. Our goodness is a gift from God, and shows his loving character. "Giving glory to God" is not an unpleasant duty, but an expression of joy. It means rejoicing in a continual relationship with God, receiving him, and participating in the wonder of his infinite goodness. We are welcomed into it, and we never have to leave.
The doctrine of original sin is a way to acknowledge the flaw ingrained in humanity, without becoming numb to its significance. It is a method for functioning within the dysfunctional human family. Without it, every sin becomes a Fall of Man, catastrophic and devastatingly unique. With it, we are invited to make forgiveness a habit of life, a regularly shining light in a darkness.