Heresy is a belief system which disagrees with the, ”One True Faith”, what ever that is. There are as many different versions of the True Faith as there are different believers though some oppressive Religions and Religious cults try to impose the beliefs of the leaders onto all members.
"A Heretic does not believe exactly what orthodoxy requires and is therefore an abomination."
Above is an intolerant viewpoint. Many different Religion and Sects are intolerant. Other religions and sects are more tolerant and tend not to use words like heretic. Typically a heretic disagrees with the orthodox believers in some ways but still belongs to the religion. Those who disagree radically and don't belong to the religion are seen as unbelievers or infidels.
Tolerance and intoleranceEdit
Some religions and some individuals are tolerant of those who disagree with them. Others are thoroughly intolerant. Historically Roman Catholics and Protestants have been imaginative in finding ways of torturing and killing those they saw as heretics as have Islammic fundamentalists.  
As with religious heresy political heresy is believing or promoting political ideas that don't fully match political orthodoxy within a political social group. The heretic is a deviant insider.  Liberals and Conservatives see each other as outsiders rather than heretics. Orthodox Republicans likely see those called Republican In Name Only as heretics. Blue Dog Democrats also aren't fully accepted among regular supporters of the Democratic Party.