heresiarch
Heresiarch (noun) means a person who starts a heresy or becomes a leading figure within one.
Etymology
Borrowed from French hérésiarque, from Ecclesiastical Latin haeresiarcha (or directly from the Latin word), from ecclesiastical Byzantine Greek αἱρεσιάρχης (hairesiárkhēs, “leader of a sect”), from Ancient Greek αἵρεσῐς (haíresĭs, “heresy”) + -ᾰ́ρχης (-ắrkhēs, “leader, ruler”), corresponding to heresy + -arch.