LogFAQs > #894609553

It's actually extremely straight forward.

Adam was a perfect man and his sin brought sin into the world. The act of sinning 'sold' the world. God felt bad for humanity, and so he made a promise with them. The promise, called the Messianic Covenant, said that a perfect man would come and sacrifice himself to 'buy back' humanity from sin. During this time, they had a bunch of rules and rituals, such as animal sacrifice, to try and seek redemption on a personal level in God's eyes (i.e; old religion). When Jesus arrived and died 'for the world's sins', the sacrifice of a perfect man meant God had bought back humanity (a perfect man for a perfect man) and now people simply had to seek redemption through Jesus instead of through complex ritual. The Jewish people, of course, believe something else (IIRC; Jesus was great, but he was not the Messiah).