As someone who looks at humans without religion, the idea of sin seems strange. Molecular biology shows that humans don't possess some inherent trait separating us from the animal kingdom. We are as much animals as the lowliest Cocolithophore to the largest Cetacean. We all come from the same common ancestor bearing what Darwin calls the "stamp of our lowly origin". Over thousands of millions of years life has struggled against overwhelming odds, the most recent, and self-aware form of cells, taking shape in homo sapiens. Everything happening today is a result of a process of change through pain, destruction, anger, love, goodness, altruism, stealing, loving, fighting, giving, killing etc. These things aren't anomalies, every creature experiences them. All we do can be provisionally explained by exposing it's survival value. Working together is better than killing each other, unless the situation calls for fighting. Sharing your food is better than hoarding, unless the situation calls for stealing. Laying down your life for someone is better than cowardice, unless a single death reaps great rewards for the masses. One just needs to search their mind for other examples.
My point is that reasons can be found for all our actions. Nothing is a mystery if we accept our origins and place as part of this world, not apart from it. Everything fits if the natural world is understood. What doesn't make sense and makes everything so unnecessarily mysterious is the introduction of the idea of sin. Sin tells us that a lot of what we do is bad objectively. That our small actions or thoughts could be so disastrous we may eternally regret them. Sexuality is abhorred, anger is shunned, coveting is rebuked, selfishness is detested, as if these actions held within them some magical property. But they don't. They are natural things animals do; we are animals. Sin is such an insidious idea that once it grips a human mind it burrows so deep it's impossible not to have it's claws dig into your brain the moment you think. It's the ultimate form of brainwashing and mind-control humans have ever invented. Once it takes hold there's no action you can take that sin doesn't make you question. What is this but servile totalitarianism, self-imposed and unalterable? William Blake's mind-forged manacles. The idea of sin clouds the moral compass and destroys any proper functioning of conscience or Socrates 'inner daemon'. The idea of sin should be purged from our collective psyche before it and all it entails drags us back to the stone-age.