Why are these ideas so hard to understand? Original sin and idea of universal damnation as the most......?

Many non-Christians identify the doctrine of original sin and the idea of universal damnation as the most repellant ideas in Christianity. Why are these ideas so hard to understand? Are the unsaved "innocent" because they do not know the laws they violate? Is God unjust in sending them to Hell? When Oedipus finally understands what he has done, he does not complain that his punishments are unjust. Why not? What is Oedipus' idea of justice?