The Original Sin

I first heard of the Original Sin in my literature class, Milton's Paradise Lost in which the writer described about Adam's disobedience to God's command. I have also read about this disobedience in the Koran but Adam has repented and God has forgiven him. Then I turned to the Bible (King James) to find out whether it says the same thing. I could not find it but instead I found the part that says that one is not responsible for another person's sin. By this logic, Christians are not responsible for the sin of Adam. We Muslims don't worry about it because we know we have to take care of our own sins only and not of somebody's. From what I understand about Christianity, the whole structure of its belief is built around the concept of Original Sin. With Original Sin gone, the structure of its belief collapses. I am not trying to discredit Christianity. I am simply intrigued by the whole thing. I am sorry if I have offended anybody