A belief contrary to authoritative religious teaching. Although neither the Bible nor the Talmud presents a systematic formulation of dogmas which followers of the Torah must believe, certain beliefs are integral to Judaism. These include, for example, belief in God, Revelation, and
Reward and Punishment.
It was not until Maimonides (12th cent.) drew up 13 Principles of Faith that Judaism had a clearly formulated list of dogmas. Denial of any of these principles, according to Maimonides, meant that the heretic had forfeited his portion in the World to Come (Afterlife). Simeon ben Tsemaḥ Duran (1361-1444), while generally agreeing with Maimonides' formulation, argued that a person did not lose his portion in the World to Come if he denied any of the principles erroneously. If he were taught incorrectly, or came to incorrect conclusions based on his own faulty reasoning, he was not a true heretic. Various writers after Maimonides offered different formulations of basic Jewish beliefs. Most of them viewed the lists of principles as effective teaching devices or as important (but not the only) statements of Jewish faith. Isaac Abravanel and David Ibn Zimra, writing in the generation of the expulsion of the Jews from Spain, argued that each and every detail of the Torah must be maintained. The denial of any point, however small, was heresy. Ibn Zimra stated in a responsum (Vol. 1, no. 344): "My mind does not agree to designate any essential principle of our perfect Torah, because it is all essential."
The Mishnah (Sanh. 10.y:1) does classify certain heresies which cause a man to lose his portion in the World to Come. These include denial that Resurrection of the dead is taught by the Torah; denial that the Torah is from heaven; holding beliefs which make one an Epikoros. Maimonides (Yad, Teshuvah 3:8) defines an epikoros as a person who denies that God communicates with humans through prophecy; who denies the prophecy of Moses; or who denies God's knowledge of the affairs of humans. The Talmud (Sanh. 99b) defines an epikoros as one who scorns rabbinic sages or who scorns others in the presence of rabbinic sages. The Shulḥan Arukh (YD 243:6) rules that it is a great sin to humiliate or hate rabbinic scholars. One who holds the sages in contempt has no portion in the World to Come. Denigration of sages leads to the mocking of rabbinic authority---and of the halakhah which depends on rabbinic authority.
Maimonides (Yad, Teshuvah 3) defines other terms which refer to individuals who hold incorrect beliefs. A Min ("sectary") is one who denies the existence of God; or believes in more than one God; or that God is corporeal; or that God is not alone eternal; or worships stars or other objects as mediators between himself and God (cf. Sanh. 38b-39a). A mumar is one who consistently and intentionally does not observe a Torah commandment, acting as though it does not exist. A broader category of mumar includes one who abandons the Jewish religion under duress, stating that it is better to join the stronger group than to remain part of the oppressed Jewish people. A kofer is one who denies the Torah, resurrection of the dead, and the future coming of the Messiah.
In rabbinic literature, a person guilty of heresy is frequently referred to as kofer ba-ikkar, a denier of a basic principle. The term is often used generically to designate an individual whose beliefs or actions reflect disregard for the Divine origin of the Torah and for the halakhah. It is incorrectly used to denigrate someone whose opinion differs from that of the defining group---even though that person's opinion has ample support in tradition. For example, in some fundamentalist Jewish circles, the term epikoros is used for a student of secular wisdom---even though he accepts the 13 Principles of Maimonides and the other basic Jewish beliefs.