Heresy is a big deal. In Western Christianity, heresy is a religious belief that has been formally denounced by any of the ecumenical councils recognized by the Catholic Church—conferences of theological experts from around the world that meet to settle issues of Church doctrine.
Throughout the history of the Christian Church, these councils have declared many beliefs heretical, and for good reason. Those beliefs which undermine the core doctrines of Christianity take believers far from the truth of God, and from the ability accurately know His nature. At their worst, these wrong beliefs could result in believers following what amounts to an entirely different religion, while still believing themselves to be adhering to good and true doctrine.
Some heresies, however, just aren’t that bad. The pursuit and elimination of heresy hasn’t always been a motivated by pure theology—history, culture, finances, and even grudges have sometimes played just as much a part. And sometimes, people simply, as they say, make a mountain out of a mole hill, turning a small error into a heresy when it could have been a teachable moment.