The doctrine of original sin teaches that all humans are inherently guilty of sin, and receive a corrupted human nature, due to Adam's disobedience in the Garden of Eden (Genesis 3). His fall from spiritual (moral) perfection, it is taught, brought death to not only himself but also all humanity.
"Adam and Eve transmitted to their descendants human nature wounded by their own first sin and hence deprived of original holiness and justice; this deprivation is called "original sin" . . . As a result . . . human nature is weakened in its powers . . . and inclined to sin" (Catechism of the Catholic Church, section "The Fall," statements 417 - 418).
"Adam by his fault transmitted to us not only death but also sin . . ." (1913 Catholic Encyclopedia article).
Where Did It Come From?
The Roman Catholic Church claims its teaching on original sin rests on the words of the Apostle Paul.
"All men are implicated in Adam's sin, as St. Paul affirms (in Romans 5:12, 19) . . . the Church has always taught that the overwhelming misery which oppresses men and their inclination towards evil and death cannot be understood apart from their connection with Adam's sin and the fact that he has transmitted to us a sin with which we are all born afflicted . . ." (Catechism of the Catholic Church, statements 402 - 403).
"That this doctrine existed in Christian tradition before St. Augustine's time is shown by the practice of the Church in the baptism of children" (1913 Catholic Encyclopedia).
The term "original sin" is not found in the Bible. It was created by Augustine who lived from 354 to 430 A.D. (Concise Theology: A Guide to Historic Christian Beliefs). In fact, it wasn't until his writings that the doctrine was fully formed (Oxford Dictionary of the Christian Church, page 994).
Reasons
There are many reasons why the doctrine of original sin is contrary to sound Biblical teachings. One of the strongest rebuttals regarding its validity lies in a careful examination of the Scriptures believed to support it!
Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, FOR THAT ALL HAVE SINNED . . . For as by one man's disobedience many were made sinners . . . (Romans 5:12, 19, KJV).
Adam's penalty for eating the forbidden fruit was being cut off from God (Genesis 3:22 - 24) and eventually suffering a physical death (2:16 - 17). Because he represented all humans, all those after him (us) would also be separated from the source of truth and righteousness.
Adam's act guaranteed that "many were made sinners" due to each person's human nature, without God, deciding for itself what was right and wrong. According to Paul's verses above, we humans do not die because of some original sin (or penalty) inherited from Adam (see Deuteronomy 24:16, 2Kings 14:6, Ezekiel 18:4, 20). We die because we also disobey our Creator!
Insights
Adam and Eve were not morally (spiritually) perfect upon creation, as such perfection requires character built by choices which they had yet to make.
Our first parents were created with the same kind of carnal and self-defeating human nature each of us possesses today! This is why they so easily and quickly justified their actions in their hearts and succumbed to temptation (Genesis 3:1 - 6). It is additionally why Adam represented all humans in what he did.
It is a gross error to pin "original sin" on Adam or any human. The first created being to have disobeyed God is Lucifer! He not only rebelled against his Creator (turning himself into Satan the devil), he did so with such zeal and single-minded dedication that he sealed his character into doing evil. He is as dedicated to doing what is wrong as God is to doing right. He is the true originator of all rebellion against righteousness.