Belief in a doctrine held to be false by the Christian Church. During the Middle Ages it was believed to be necessary to follow the one ‘true’ religion, which provided the only guarantee of salvation and afterlife. Consequently those who came to believe that orthodox teaching was inadequate or wrong risked being declared heretics. Since the Church sought to maintain the unique validity of its declared doctrine conflict was inevitable.

The early Church condemned Gnostics in the 2nd century and Arianism and the Nestorians in the 4th century. The Iconoclasts were condemned at the Council of Nicaea in 787. The condemnation of the Cathars in southern Europe led to the Albigensian Crusade. Later dissatisfaction with orthodox teaching led ultimately to the establishment of the Protestant Church. The Inquisition from its earliest days upheld the Church’s doctrine and became responsible for the rooting out of unorthodoxy.