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People considered heretical in faith were named by various terms in Jewish literature. Jewish scholars, especially Maimonides (d. 1204), categorized heretics according to the kind of heresy. Later, they included Jews who had similar thoughts and behaviors in the same category and warned them about God’s punishments for heretics. One of the standard terms used in Jewish literature for heretics is apikoros. The term apikoros was derived from the Greek philosopher Epicurus (d. 270 BC). Although it was a proper name at first, it gradually turned into a common name denoting people who were unbelievers or imperfect in their faith. Having thoughts that contradicted the basic principles of the Jewish faith, the apikorsim became a group against which the Jewish clergy sided. Because of the fact that they misled Jews from the right path, many discussions arose in Jewish literature, such as how to struggle apikoros. This article discusses the origin of the term of the apikoros, the mindset of the apikoros, their effects on the Jews, their position in the Jewish society, and the punishments they will receive in the hereafter.