The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), in his list of sinners who forfeit their share in the next world, includes "Moserim," or "informers." He writes (Hilchot Teshuva 3:12; listen to audio for precise citation) that this category refers to Jews who provide information about a fellow Jew to the gentile authorities or to criminals which allows them to kill the Jew or cause him physical or monetary harm. Regardless of whether the gentile kills the Jew, beats him, or confiscates his property, the Jew who provided the gentile with the information is deemed a "Moser" and has no share in the World to Come.
In the next passage (Halacha 13; listen to audio for precise citation), the Rambam introduces the next category of sinners who have no share in the next world – "Matil Ema Al Ha'sibur She'lo Le'shem Shamayim." This term refers to people who impose dictatorial rule over the Jewish community for self-serving and self-aggrandizing purposes. Their motives are not for the sake of promoting Torah observance, but rather purely for their own honor. Such people, the Rambam rules, have no share in the next world.
The Rambam concludes his list by enumerating several "lighter" transgressions for which a habitual violator loses his share in the World to Come. This list includes people who publicly humiliate their fellow, who disrespect Torah scholars, who call others with derogatory nicknames, who receive honor through the downfall and shame of their fellow, who do not treat the festivals with the proper respect (some explain this as a reference specifically to Hol Ha'mo'ed), and who show disrespect to Kodashim (Temple property). People who regularly engage in such conduct lose their share in the next world.
The Rambam concludes this discussion by emphasizing that he speaks here only of sinners who do not repent. In all the cases listed of sinners who forfeit their share in the World to Come, the individual can earn his portion of the eternal life through the process of Teshuva (repentance). The Rambam goes so far as to say that even a person who denied God's existence his entire life can earn a share of the World to Come if he sincerely repents before his death. Even if the sinner repents privately, without the knowledge of any other person, he can earn atonement and regain his share in the next world.
