Original Sin, freedom and liberty
The first man, when God created him, he didn't know evil and, therefore, could only act in a good way, according to God's law. Know evil implied man would have to choose good instead of evil. That would be real freedom. There was a tree, with would give man the knoledge of good and evil. God forbade man eating it's fruits. But the devil convinced Eve and Adam to do so, believing they'd become superior to their creator. That's the original sin, and we all have to be cleaned from it if with baptism.