The doctrine of sin and the doctrine of atonement are correlative in the same way that a disease and its remedy are correlative. If sin is that from which Christ saves us, then the strength of the remedy (atonement) must vary according the severity of the disease (sin). As a first approximation, a sinful act can be thought of as a morally bad act for which the sinner is responsible. But the language of “sin” adds something to the language of moral wrongdoing: a sin is a failure or fault with respect to God. Like other Christian doctrines, the doctrine of sin poses tricky philosophical problems. To see those problems more clearly, it is useful to disambiguate the doctrine of sin into several distinct components: the first sin, the Fall, original sin, and personal sin.