Still, the Mishna in Sanhedrin 90a makes clear that there are a handful of crimes so terrible that they can cause a person to forfeit his share in the World to Come. Significantly, these are not crimes against fellow human beings or infractions of Torah law; rather, they are what George Orwell called thought-crimes. Specifically, they involve dissent from what the rabbis regarded as the core doctrines of Judaism: “And these have no share in the World To Come: One who says, There is no resurrection of the dead derived from the Torah, and one who says, the Torah is not from Heaven, and an epikoros.” An epikoros is a mocker of religion, someone who denies honor to God and to the rabbis, God’s representatives. It’s not clear whether the truly culpable thing here is holding these heretical opinions or “saying” them aloud, trying win adherents for them. But in any case, the rabbis make clear that a person who believes these things is in some essential way not a Jew.Incidentally, the debate about whether the doctrine of resurrection is found in the Torah (it isn't, although there have been many efforts to read it in) also comes up implicitly in a discussion between Jesus and the Sadducees in Mark 12:18-27 and parallels.
Also, as this essay goes on to note, Rabbi Akiva is reported to have classed reading "external literature," that is biblical apocrypha and pseudepigrapha, as heretical.
Earlier Daf Yomi columns are noted here and links.
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