The origin of the idea of "original sin"
The term "original sin" has more than one useage. One useage refers to the sin committed by Adam and Eve in Genesis, the firsr book of the Bible; Genesis states that they they disobeyed the command of God, and ate the fruit of the tree of good and evil. After this God expelled them from the Garden of Eden. According to the classical Judaism understanding of this story, the consequence of this action was to both make man mortal, and also aware of the consequences of his/her actions (i.e. humanity gained free will).
Original sin in The New Testament
The concept of original sin underwent development by Paul, in Romans and First Corinthians, in the New Testament. Paul placed special emphasis on this by stressing that belief in Jesus would allow Christians to overcome death, by earning salvation in the hereafter. The New Testament teaches that rejection of Jesus as the path to salvation must be viewed as willful disobedience, and a rebellion against God. This choice then compels a just God to enforce that person's separation from Him, causing such a person to be sentenced to Hell, or in some views, Limbo. Only belief in Jesus, as a savior and son of God, could rescue a person from this fate.
Augustine's modern formulation of original sin
Under Augustine the common and modern-day understanding of Original sin was formulated; he taught that the taint of Adam's original sin was inherited by all people at birth, and and that nothing a person does in their life can get rid of this taint. This doctrine took on special prominence in Catholic Christianity and in many Protestant Christian denominations.
In most branches of Christianity, the doctrine of Original sin states that all humans have inherited the guilt of sin from Adam and Eve; this state of sin exists in all people from the moment of their conception. According to this doctrine, all people are born sinners and die sinners; all people are 'lost' eternally, and are in need of Divine salvation. The only way people can be justified in God's eyes and reconciled with God is by humbly asking for forgiveness, believing that His son Jesus Christ, through his death and crucifixion, took on himself the due punishment for our sins and trespasses (atonement), and living life in obedience to God. The ultimate punishment for the original sin was expulsion from the presence of God and subjection to physical and spiritual death; the ultimate goal and blessing of reconciliation is the restoration of the original relationship man had with God; this includes eternal life.
Christians have different views on the way to receive salvation from original sin. On one end of the spectrum are they who believe that only a chosen elect, the predestined, will come to realise their fortunate position, while all the others will have to suffer under the yoke of sin and never achieve any assurance of salvation. On the other end are they that believe that every person ever born will ultimately be justified, restored and saved. Between those two poles are those that believe that people are still free to choose life with God or separation from God; people remain dependent on God's grace and mercy, but also have a role in participating in their salvation.
Original Sin as understood by Orthodox Christianity
Augustine wrote in Latin in the fourth century, but his writings were not translated into Greek until the fourteenth century. Consequently, Eastern and Oriental Orthodox Christianity never held that guilt is inherited, and began repudiating this idea once they learned of it. They teach that we inherit a corrupted or damaged human nature in which the tendency to do bad is greater, but that each person is only guilty of their own sins. By participating in the life of the church, each person's human nature is healed and it becomes easier to do good; at the same time, the Christian becomes more acutely aware of his or her shortcomings. Eastern Orthodox theologians believe that Adam and Eve began to choose separation from God when they choose independence and took fruit for themselves, rather than allow God to continue to feed them and remain dependent on Him. The expulsion from the Garden was not a legal consequence, but to prevent them from eating of the Tree of Life and immortalizing their sin. As Christians partake of the Eucharist and eat and drink the Body and Blood of Christ, they return to dependence on God and experience a gradual healing of the relationship between God and humanity.
Meaning of the story
Some people attribute their expulsion from Eden as punishment for disobeying God's commandment (Gen. 2:17) "Of the fruit of the tree of the knowledge of good and evil you shall not eat . . ." Others interpret the fruit as the symbol of something so precious that Adam and Eve would risk their lives to "eat of it."