Fun fact: the head of a heretical movement is a heresiarch. Anyway, the word heresy has been used for a while to describe beliefs contrary to the Church, whatever religion that might be. This was borrowed in the early days of Middle English from the Old French word heresie, which described immorality of any kind. In Latin as haeresis, this referred to a particular philosophical notion that we should choose to be immoral, and that choice becomes all the more important as we trace this further back to the Ancient Greek word hairesis, meaning "choice". This is from the verb harein, which meant "to choose" and officially has an unknown origin. However, speculation can bring us back even more to the Proto-Indo-European reconstruction ster, which could have meant something more like "to seize" or "to take". So all this means that heresy is a choice, a philosophy you happen to take. Unsurprisingly, use of the word heresy has decreased since conservative times.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd