This morning I got into a discussion about sin with a commenter at TitusOneNine. Here's an edited version of what I said.
As to 'sin,' certainly we're all sinners in that we all make 'bad choices.' But we understand very little about why and how we make any choices at all, bad or good.
Traditional Christian models of 'sin' don't offer us much insight here. And when it comes to the acid test, that of combating sin (let alone all but wiping it out as we have, say, polio), traditionalist sin models have an uneven track record, to put it politely.
Combating sin with prescriptions derived from traditionalist models is analogous to the way doctors used to treat peptic ulcers with diet changes and stress reduction. In the case of peptic ulcers, sometimes the treatment worked, sometimes it didn't. The trouble was, doctors could never reliably predict which it would be for a given patient. When the treatment did work, they didn't understand why, because they didn't understand how and why people got peptic ulcers in the first place.
It's much the same with the traditionalist 'treatment' for sin. By this I mean the usual prescription of accepting Jesus into your heart, believing that he was God incarnate who died for our sins, and so on. Sometimes the treatment works, but very often it doesn't. We can't reliably predict which it will be for a given sinner. And when the treatment does work, we don't understand why, because we don't understand how and why people make the choices they do.
In the case of peptic ulcers, doctors eventually figured out that most common peptic ulcers are caused by a particular type of bacteria, Helicobacter pylori. The standard treatment is now an inexpensive course of antibiotics.
In the analogous case of sin, we've got a lot of work to do to reach that level of understanding. I would venture that many nonbelievers and doubters instinctively understand this, and for that reason (among others), exclusivist claims about Jesus strike them as foolish nonsense. This certainly doesn't help our efforts to bring such people to God.