Heresy is usually defned as an error concerning the content of faith. In this article heresy is shown as a sin requiring conversion and penance and not just a withdrawal of one’s views. A sin of heresy is compared to adultery or idolatry, for which the same penance used to be assigned (e.g. Synod of Elvira in 306, can. 22). In this context the condemnation of Nestorius by the Council of Ephesus in 431 is characteristic because it is focused on the insult to Jesus Christ and not on erroneous conceptions. It is also the case with the formulas of condemnation of heretics where such invectives as contamination, sacrilegium or perfdia were often used, and those terms belong to the feld of morality rather than to intellectual disputes or differences.