|The state of Sin that has captured human beings since the Fall. The idea that we have plenty to be ashamed about simply by being alive is one of the less appealing of ethical doctrines, but it is also one of the main components of any variety of Christianity, since without it the doctrine of the atonement loses its rationale.
The precise way in which Adam's guilt was transmitted to us all has, naturally, exercised theologians. For Augustine, it simply comes as a consequence of sexual reproduction and its accompanying concupiscence. For Aquinas, it is not so much sin that is passed on as the loss of a supernatural capacity to govern the lower appetites by means of reason.
Original sin is functionally necessary to dramatise the importance of redemption, and the religious practices that facilitate it.
Traditionally, Adam "ate of the fruit of the tree of knowledge of Good and Evil," having been expressly forbidden by God to do so. This act granted Adam additional capacities (the knowledge of Good and Evil), but was viewed dimly by God who expelled Adam and Eve from Eden and imposed various penalties on them both, and everyone descended from them (i.e. everyone).
Had the Fall not happened, the atonement of Jesus Christ for the original sin, and the more pedestrian variety, would not have been necessary. Given that The Christian God is thought of as Omniscient, and Jesus Christ as pre-existing, this raises some difficult questions.
In this last sense concupiscence is a thoroughly bad thing. The Pelagian heresy contained the view that concupiscence is innocent, and was enthusiastically countered by Augustine, who founded the tradition of identifying concupiscence with fleshy lust, and hence as a vehicle for transmitting original sin.