Original sin

According to Christian theology, Original sin is the state of being sinful into which all people are born. Original sin refers to the very first sin, committed by Adam and Eve. Original sin is sometimes referred to as "the Fall". St. Augustine introduced original sin to Christians.

For Christians, Paul's discussion in Romans Chapter 5, v. 12 ff is operative:

Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: (For until the law sin was in the world: but sin is not imputed when there is no law. Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come.

Elsewhere, Paul discusses the idea that Adam and Eve, as sinless creatures of God, were immortal in the flesh. The consequence of their sin was death, mortality, a sentence they passed onto all their progeny. Jesus, as the New Adam was a new, sinless creation of man; while capable of sin, Jesus never sinned, and was therefore exempt from the death penalty. But he died, and quite horribly in fact. The why of the death of Christ is wrapped up in the doctrine of the Atonement; suffice to say Christ chose suffer the penalty of human sin as an atonement for it.