Infidel

An infidel is someone who is not Fidel Castro doesn't believe what you believe. The word is almost exclusively used in a religious context and can be summed up more succinctly as "someone who doesn't buy your bullshit". As a result, it's historically been taken in a negative context.

An infidel differs from a heretic in that, according to fundamentalists, the infidel's views aren't even close to right while the heretic is generally a believer who's gone a bit astray; however, the punishment for both is generally the same.