Man was created to have an eternal relationship with God. When
man disobeyed God's instruction in Eden, Sin entered the picture.
Sin caused the relationship with God to be not possible. From that
point, man have tried fruitlessly to repair this broken
relationship.
For sin to be forgiven, sacrifice must be made. The act in Eden was
an act of rebellion; rebellion is punishable by death. However, no
sacrifice was good enough except Christ himself. Jesus was made the
sacrifice so that all man can be forgiven through Him.
That said, it must be pointed out that forgiveness needs two
parties to be involved. One to forgive, the other to receive the
forgiveness.
The above explanation is quite brief, but I hope it answer some
parts of the question.
