Old Sin Nature?
The problem is that the idea is actually false. In philosophical terms, to say something is "by nature" a certain way, means that that being MUST have that trait in order to be who or what it is. For example, human beings are by nature rational beings. Anything that has no capacity (even an undeveloped or hindered one, e.g., a fetus or a mentally retarded person) for reason or knowledge is simply not a human being. This idea can be expressed by saying that the capacity for rationality is "essential" (another philosophical term) to human beings. It is an essential property of humans.
From this, we see that the claim that humans are sinful by their very nature is actually unbiblical. For it is not essential to human beings that they be sinful. Adam and Eve, for example, had no sin when they were first created, but they were obviously human. In fact, God proclaimed them to be part of His distinctly "good" creation. Moreover, humans will not sin in Heaven. But they are human, nonetheless.
Christ did indeed die to redeem mankind from its sin. But this doesn't require the idea that man is essentially sinful. Rather, sin is a deadly parasite on human nature and can only be removed by the blood of Christ. It is only then that our true, unblemished natures can ultimately be revealed.
