original sin, in Christian theology, the sin of Adam, by which all humankind fell from divine grace. Saint Augustine was the fundamental theologian in the formulation of this doctrine, which states that the essentially graceless nature of humanity requires redemption to save it. The purpose of baptism is to wash away original sin and to restore the individual to an innocent state, although even after baptism a tendency to sin remains as a result of original sin.

In Christian doctrine, the condition or state of sin into which each human being is born, or its origin in Adam's disobedience to God when he ate the fruit of the tree of knowledge of good and evil. His guilt was transmitted to his descendants. Though Genesis describes Adam's sufferings as the consequence of his disobedience, it does not make Adam's sin hereditary. The main scriptural basis of the doctrine is found in the writings of St. Paul; St. Augustine helped make humanity's sinful nature a central element in orthodox Christian theology. Medieval theologians continued to explore the issue, and Thomas Aquinas offered a more optimistic view of human nature in his teachings on original sin than had Augustine. Martin Luther and John Calvin accepted a more Augustinian understanding, and modern evolutionary theory and biblical source criticism raised new challenges for the definition of original sin.

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