II. History of the doctrine
Although the concept of original sin is derived from the story of Adam and Eve's disobedience recorded in Genesis, the term "original sin" and the concept of a hereditary sin passed on to the entire human race are totally absent from the Old Testament and the gospels. Jesus is not recorded as ever having mentioned original sin, and Genesis relates only that the sin of the first parents brought consequences upon them.
The dominant theological understanding of humankind in Old Testament Israel is that man is sinful.  There are numerous passages in the Old Testament on the universal sinfulness of all persons, such as the rhetorical questions: "If you, O Lord, should mark iniquities, Lord, who could stand?" (Psalm 130:3) and "Who can say, 'I have made my heart clean, I am pure from my sin'?" (Proverbs 20:9).  Moreover, the great flood was God's punishment for "the wickedness of humankind." (Genesis 6:5) This sinfulness, however, was not viewed as the consequence of one man's action at the beginning of history. 
Passages in the Old Testament "appear to speak of a sinfulness that is innate; but this innate sinfulness is to be understood as an innate proneness to sin rather than as an inherited state of sinfulness. And, most important of all, it is not connected with the fall of the first parents as its origin." 
In the New Testament, only Paul writes about Adam's sin and Christ's redemption: "For just as by the one man's disobedience the many were made sinners, so by the one man's obedience the many will be made righteous." (Romans 5:19)
The theology of original sin developed out of questions that arose in the third century concerning the custom of infant baptism. St. Augustine (354-430 A.D.) is credited with developing the traditional doctrine in response to Pelagius, who denied original sin. Augustine appealed to Scripture to blame Adam for original sin and to the existing practice of infant baptism to defend the idea that the sin is passed on to all Adam's descendants -- an idea subsequently endorsed by St. Thomas Aquinas. 
Original sin was taught by the Council of Carthage in 418 A.D.  and the Second Council of Orange in 529 A.D.  The doctrine was formally defined by the Council of Trent in its Decree on Original Sin (1546 A.D.). 
More recently, Pope Pius XII asserted:
Original sin is the result of a sin committed, in actual historical fact, by an individual man named Adam, and it is a quality native to all of us, only because it has been handed down by descent from him. 
The Catechism of the Catholic Church affirms original sin as "an essential truth of the faith."