In the third chapter of Hilchot Teshuva, the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) delineates the groups of sinners who forfeit their share in the World to Come. In Halacha 8 (listen to audio for precise citation), he defines the group known as "Apikorsim" ("heretics"), explaining that this category includes three types of people:
1) Those who deny the concept of prophecy, that God delivers instruction to human beings.
2) Those who deny the prophecy of Moshe Rabbenu. Even if one accepts the notion of prophecy generally, he has no share in the next world if he denies the prophetic stature of Moshe.
3) Those who deny God's knowledge of human events and experiences.
All these people fall under the category of "Apikorsim" and have no share in the World to Come.
The next category discussed by the Rambam is that of "Kofrim" ("rejecters"), and consists of people who maintain one of the following three beliefs:
1) That the Torah was not transmitted in its entirety from God. Even if one denies the divine origin of a single verse or a single word of the Torah, and claims that Moshe wrote it independently, without it being dictated by the Almighty, he loses his share in the World to Come.
2) That the Rabbinic tradition of Torah She'be'al Peh (oral tradition) does not originate from God. Even if a person accepts the divine origin of the written Law, he is deemed a "Kofer" if he rejects the traditions of the Sages in explaining and applying the written Torah.
3) That the Torah or any part thereof is no longer applicable, or has been supplanted by a different law or system of laws. Christians, for example, believe in the divine origin of the Torah but claim that it has since been replaced by a new law. Anybody who maintains such a belief, or says about even a single Misva that it has been repealed or substituted with another law, has no share in the next world.
In Halacha 9 (listen to audio for precise citation), the Rambam proceeds to define the next category of sinners, the "Meshumadim" ("defectors"). He writes that this term refers to one of the following types of sinners:
1) A person who intentionally, habitually and publicly rejects one of the 613 commandments. Even if a person faithfully observes the other 612 Misvot, if he makes a point of publicly transgressing one Misva on a regular basis, such as if he intentionally wears Shaatnez (a garment woven with wool and linen) or cuts his sideburns, he is deemed a "Meshumad."
2) A person who chooses to abandon Judaism and embrace the religion of an enemy people that enjoys the upper hand. If a person defects from Judaism in order to spare himself the humiliation and persecution traditionally suffered by Jews, and to enjoy the benefits of belonging to the ruling people, he is deemed a "Meshumad" and has no share in the World to Come.
