Heresy
Heresy is the act of speaking against the word of the an established religious organization, or simply having private beliefs that are not in accordance with the officially-endorsed position.
In early Christianity, heresy was a recurring problem as people pretending to be Christians would advocate doctrines contrary to the Bible. As heresy was seen as a great threat to the church, spurious allegations of heresy were also used by rival ecclesiarchs to attack their enemies. A series of Councils were held to establish official doctrine, in some cases leading to schisms, such as the split between the Roman Catholic and Eastern Orthodox Churches in the eleventh century.
Today, accusations of heresy are not as widespread as they once were, but some Christians may privately consider those of other denominations to be heretics.