A heretic is someone who publicly disagrees with or rejects the official beliefs of their religion. In medieval Europe, if a Christian openly denied a core teaching of the Catholic Church, like questioning whether saints could perform miracles, church authorities might declare that person a heretic. The word carries a sense of dangerous rebellion against accepted doctrine.

For centuries, being labeled a heretic was extremely serious. Religious authorities feared that heretical ideas would spread and weaken the faith of others. During the Middle Ages and Renaissance, people accused of heresy faced trial, and some were even executed. The scientist Galileo was tried for heresy when he argued that the Earth revolves around the sun, contradicting the Church's teaching at the time.

Today we use heretic less dramatically, sometimes even playfully. A basketball fan who insists that a new playing style is better than traditional methods might jokingly call themselves a heretic among purists. A chef who adds pineapple to pizza might be called a heretic by Italian food lovers.

The related word heresy means the belief or opinion itself that goes against orthodox teaching. What counts as heresy depends entirely on who has the power to define orthodoxy, whether in religion, science, or even sports fandom.