The concept of sin is introduced in the first few pages of the Old Testament and revisited frequently throughout the entire Bible. It is very much a foundation stone upon which Christianity is built.
Indeed if one doesn't accept the sin element described in the Old Testament, there is almost no point in the character of Jesus in the New Testament. After all, Jesus bravely died for your sins on the cross.
Philosophically I have a problem with the whole concept of sin. What makes something sinful?
I decided to ask a few Christian friends how the moral baseline is drawn. I asked them all the same question:
“Do you consider certain things to be intrinsically sinful? For instance: is murder a sin, just because it is? Or, do you think sin is defined by society, and evolves? For instance, homosexuality was once considered sinful, but society has now forced us to reconsider and it's arguably less sinful now than it once was. Or as a Christian, do you think God defined what is sinful and what isn't -  when he created everything -  and that definition never changes?”
In fact I asked three christians. One declined to answer. One was very positive that God defines sin and that it can't change. The other danced around the topic for hours before deciding that God reminds us what sin is in the Old Testament, but that society certainly has a role in defining sin, and that it can evolve within the boundaries set down by God.
It seems fairly astounding to me that even Christians can't exactly put a finger on where or how the line is drawn, but I admit my poll wasn't very scientific. The atheist has no such problem in any case. We all draw our own moral line in the sand based on what we learn as we journey through life. It's not fixed. We can rub that line out any time we want and re-draw it.
If God does define what sin is, doesn't that raise a few issues? For a start, we're not in a position to change the boundaries. If God says you can't do it, you can't do it. And, rather embarrassingly,  God says you can't be homosexual, or get divorced, or eat shellfish. Conversely he also says that slavery, within certain limits, is perfectly acceptable. If those rulings can't change as our collective wisdom grows, then we're not really making any value judgements; we're just following rules. That's not a moral code is it?
But if God doesn't define what sin is, if we're supposed to decide by ourselves, then why does God give us his guidelines on sin in the Old Testament?  That's just going to encourage ludicrous nutters like Kim Davis and the Westboro Baptist church.
All Christians cherry-pick from the Bible and disregard what they consider to be unimportant. Homosexuality seems to be one of those sins often considered important. Divorce on the other hand appears to be a sin which we can largely ignore today. As such I think the average Christian probably doesn't truly accept that sin is defined by God, despite what they might say.
I don't believe anything is inherently sinful. If something is deemed hugely sinful now, but was once quite acceptable- for example, slavery, there can't be anything about slavery itself that makes it morally wrong. It has to be the changing environment or social factors that made it so.
All this makes me wonder how Christ is supposed to have died for my sins. Apparently even defining where the line is drawn is non-trivial. Perhaps the definition of sin itself doesn't really matter because as long as we accept that Christ died for our sins on the cross, we are absolved anyway.
Except, I genuinely don't understand how that works. If I murder someone, and somebody else takes the blame, stands up in court and confesses, goes to jail, maybe even takes the death penalty for me- that doesn't change history. No one can take away my responsibility. I still murdered someone.
Even a deathbed conversion to Christianity gives me a "get out of jail free" card, regardless of the life I've been living up to that moment. That's a revolting concept isn't it? I can be a foul and immoral person for 70 years and then convert the moment before I die to attain eternal salvation. What kind of moral lesson is that?