Heresiarch combines two elements taken from relatively provocative words: heresy and anarchy. The first part of the word, ‘heresi-‘ is indeed related to ‘heresy’ and retains its modern meaning in this context. However, its root is the Ancient Greek word, haíresis, which means ‘to choose’. This lends an interesting note to the word in that it seems to indicate that one who chooses to profess a view that goes against the existing orthodoxy risks being labeled a heretic. It is an oddly value neutral root for a word that in its modern context is heavily laden with negative meaning.
The second part of the word, ‘-arch’ doesn’t come from ‘anarchy’, but is rather the base on which that word is built, the Ancient Greek term arche, which means something like ‘one who begins’. A heresiarch is therefore the originator of an unorthodox idea.
It is notable that the label of being a heresiarch has tended to be assigned from the perspective of the religious establishment (therefore it is applied to religious ‘rebels’) but only to those whose movements were not successful. So, while Jesus and Martin Luther, for example, who could be said to have bucked orthodoxy and revolutionized the religious landscape of Western civilization, are not labelled heresiarchs (because their respective sides ‘won’ to the extent that their spiritual movements were not defeated), someone such Arius the Heresiarch, whose ideas and movement were defeated, retains the taint the label to this day.
Arius was a North African Presbyter (equivalent to a modern priest) in the early centuries AD who attended the Council of Nicaea in 325 AD. The Council had been called by the Roman Emperor Constantine to standardize the doctrine of the early church, and the view championed by Arius which, in essence, denied the idea of the holy trinity was defeated and fell out of favor. The lack of favor led to this viewpoint, called Arianism, being suppressed and eventually destroyed, and the man himself being labelled an official heretic by the church. Arianism as a creed was dropped by many over the next few hundred years, and in those areas of stubborn resistance, military force was used to bring the population into the ‘trinitarian’ fold. In short, Arian lost the battle, thereby making the views and positions he had ‘chosen’ heretical.
In some ways it is surprising that a word as interesting as heresiarch is used so infrequently, but I believe that two factors act to restrain its usage. First, it rose to prominence in a religious context and there seems to be a cultural hesitancy to use the word outside of this context for fear of breaking an unspoken taboo or denigrating the seriousness of the religious connotation of the word by using it in a profane setting. Second, as Western society becomes more secular and relativist, the identification of heresies and, therefore, heresiarchs has become infrequent.