Someone who is anti or who hates or persecutes all forms of religeous beliefs or non beliefs including Atheists and Agnostics.
"I see that an Anti-Theist has flown a plane into a tall building" "Er, surely you mean an Islamist?" "Er no, where does it say in the Queran that it is ok to go doing stuff like that?" "Well ok, but surely you mean Satanist?"
"Point taken...But can we settle for, an Anti-Theist is just a Satanist who hasn't yet discovered themself?"