I think I know why Pagans and Heathens are hated more than Atheists and Agnostics by Christians.
It's easy to shake someones lack of faith in something, but shaking someones faith in any religion is a very difficult task. If a Christian REALLY wants to convert someone, an agnostic explicitly does not have a faith to shake. In the meantime, Heathens build their entire worldview and religion by studying things themselves.