Original sin is the Christian doctrine that holds that humans, through the fact of birth, inherit a tainted nature in need of regeneration and a proclivity to sinful conduct. The biblical basis for the belief is generally found in Genesis 3 (the story of the expulsion of Adam and Eve from the Garden of Eden), in a line in Psalm 51:5 ("I was brought forth in iniquity, and in sin did my mother conceive me"), and in Paul's Epistle to the Romans, 5:12-21 ("Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned").

The belief began to emerge in the 3rd century, but only became fully formed with the writings of Augustine of Hippo (354–430), who was the first author to use the phrase "original sin" (Latin: peccatum originale). Influenced by Augustine, the councils of Carthage (411–418 AD) and Orange (529 AD) brought theological speculation about original sin into the official lexicon of the Church.

Protestant reformers such as Martin Luther and John Calvin equated original sin with concupiscence (or "hurtful desire"), affirming that it persisted even after baptism and completely destroyed freedom to do good, proposing that original sin involved a loss of free will except to sin. The Jansenist movement, which the Roman Catholic Church declared heretical in 1653, also maintained that original sin destroyed freedom of will. Instead, the Catholic Church declares that "Baptism, by imparting the life of Christ's grace, erases original sin and turns a man back towards God, but the consequences for nature, weakened and inclined to evil, persist in man and summon him to spiritual battle", and that "weakened and diminished by Adam's fall, free will is yet not destroyed in the race."