Original Sin

Original Sin is a Christain Doctrine that states that every Human is evil, wicked, corrupted and heinous unless Baptised, simply for being born.

Origins
The Original Sin comes back to the Genesis Creation Myth in which Adam in the Garden of Eden ate an apple from an evil tree (the reason why it was there in the first place isn't fully explained) that homes a talking snake (again, reason for presence not explained) that convinced Eve to eat the apple because all she wanted was to know right and wrong. Since God told them not to, he had them clothed and kicked out of the Garden of Eden as well as giving Eve (the representation of all women) the pain of birth. That one sin then reverberated on all of humanity, causing God to commit global genocide years later, flooding the world and killing everyman, woman, child and animal except for Noah and his extended family.

Thousands of years later, God found the will to forgive us of our inherited sin by sending his one and only son to Earth, Jesus, and having him tortured and crucifed for our sins.

Contradiction
Since the existence of Adam and the whole Genesis event was disproven by the impressively intelligent Theory of evolution, the whole concept of Original Sin is wrong. Many modern mainstream-Christians who believe in evolution claim the that Genesis was simply symbolic, which pretty much makes the sacrifice of Jesus pointless (though probably either way).