Original sin is the doctrine which holds that human nature has been morally and ethically corrupted due to the disobedience of mankind's first parents to the revealed will of God.
In the Bible, the first human transgression of God's command is described as the
sin of Adam and Eve in the
Garden of Eden resulting in what theology calls the Fall of mankind. The doctrine of original sin holds that every person born into the world is tainted by the Fall such that all of humanity
is ethically debilitated, and people are powerless to rehabilitate themselves, unless rescued by God.
There are wide-ranging disagreements among Christian groups as to the exact understanding of this doctrine, with some so-called Christian groups denying it altogether. Eastern Orthodoxy, Judaism, and Islam acknowledge that the introduction of sin into the human race affected the subsequent environment for mankind, but deny any inherited guilt or necessary corruption of man's nature.
However, G. K. Chesterton once noted that "Certain new theologians dispute original sin, which is the only part of Christian theology which can really be proved," (Orthodoxy, chap. 2). He saw original sin as the one Christian doctrine that is empirically verifiable and validated by 3500 years of human history.
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Some hold that humans are only responsible for conscious acts of sin:
"We believe that original sin differs from actual sin in that it constitutes an inherited propensity to actual sin for which no one is accountable until its divinely provided remedy is neglected or rejected."  This view maintains that the effects of original sin are "innocent effects", and that people are only held accountable for "actual sins" or "personal sins", which are a "voluntary violation[s] of a known law of God by a morally responsible person." 
- ". . . all men, born according to nature, are born with sin, that is, without the fear of God, without confidence towards God and with concupiscence, and that this original disease or flaw is truly a sin, bringing condemnation and also eternal death to those who are not reborn through baptism and the Holy Spirit" (Article II). - Augsburg Confession of Faith (1530)
- Henri Blocher, Original Sin: Illuminating the Riddle. NSBT. IVP Academic, 2001. ISBN 083082605X
- John Murray, The Imputation of Adam's Sin. P. & R. Publishing, 1977. ISBN 0875523412
- Jonathan Edwards, "The Great Christian Doctrine of Original Sin Defended" in The Works of Jonathan Edwards Volume I, pp. 143-233.