As Rav Yaakov Shapiro use to teach, a Nebach apikorus is just an unintentional apikorus–i.e. a tinok shensiba who wasn’t taught the 13 ikarim. A regular apikorus is an intentional denier of either the 13 ikarim or any other part of the torah. (An unintentional denier/mistake of any part of the torah outside the 13 ikarim would just make the persons ideas an apikorus without that person himself acquiring that status)
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As from my notes of his:
1) Apikorsus (a.k.a. “meenus” a.k.a kefirah – the words are often used interchangibly) is anything that is contrary to the Torah’s opinion, regardless of whether it contradicts one of the 13 Ikarim or not.*
2) Not everybody who says Apikorsus is himself an Apikores. As the Chazon Ish writes about a piece of apikorsus he saw in a certain sefer: “Even though the author was a Yorei Hashem, he worshipped Apikorsus unwittingly.”
3) To be a full fledged Apikores yourself, which means we have no mitzvah to love you, – on the contrary, there is a Mitzvah to loathe you (“l’snoso”); we don’t return your lost items; your wine is yayin nesech; you lose your share in Olam Habah; you are, for all practical purposes, not part of klall yiaroel, and you have the status of a goy regarding all privileges (even though you are still obligated to fulfill all the Mitzvos) – for that, you have to disagree with one of the 13 Ikarim, regardless of whether you are doing so on purpose or out of ignorance. That is, even if you dont realize that you are disagreeing with them, you are still a full fledged Apikores, as Rav Chaim Brisker ZTL used to say: Nebach an apikores is oich an apikores “Someone who is an Apikores through no fault of his own is an Apikores nonetheless”. If you knowingly disagree with anything the Torah says, meaning, you know the Torah says such-and-such, and then you say “The Torah is wrong,” you are in this category as well.
4) Even if you do not actually deny one of the 13 Principles, but merely are unsure about it, you are still a full fledged Apikores. You must be convinced of the absolute truth of these 13 Principles in order to get your share in Olam Habah and be considered part of Klall Yisroel, as per #3 above.
5) There is a disagreement between the Rambam and the Raavad regarding someone who, in the course of his Torah learning, makes an honest mistake, misunderstands something he sees in the Torah, and erroneously derives from the Torah a belief that is actually Apikorsus. The Rambam holds this person is an Apikores and the Raavad holds that he is not. **
5a) Even the Ravad, however, agrees that even though the person himself is not considered an Apikores, his mistaken belief is indeed considered Apikorsus.
6) Rav Chaim’s rule of nebach a apikores is also an apikores. A nebach apikores is a like a tinok shenisba who is not cognizant of the 13 ikarim.
7) Even Orthodox people who fulfill all of Torah and Mitzvos can be full-fledged Apikorsim. Said the Chazon Ish, “There are people whose homes are sufficiently kosher that you may eat the meat, the fish, and all of their food, but there’s only one food item you can’t eat: You can’t drink their wine, because their Hashkofos make it yayin nesech”. Even great Talmidei Chachamim can be Apikorsim. The Rambam states that even if a person has great wisdom and has great merits due to his Torah knowledge, he is still a full-fledged Apikores if he does not believe in the 13 Ikarim.
8) There is a belief held by many people that you have to know the whole Torah, or at least some great measure of it, in order to be an Apikores. This is a myth. There is no such thing. As long as you deny one of the Yesodei Hadas, one of the Fundemntals of our Religion, you are an Apikores. There is also a myth, believed by many, that there is “no such thing” as an Apikores nowadays. This, like the “kol hatorah kulah” myth, is a fairy tale. (Historically, they both cropped up as baseless rationalizations of religious Zionists who befriended and sometimes even raised to the stature of heros, athiest Zionist Apikorsim. Through these wishful ideas, they attempted to remove those atheist enemies of G-d and Torah from the category or Apikorsim)
8a) In order to preempt what I have heard on more than one occasion cited as “a source” for the above, the Chazom Ish does not say anywhere any such thing. On the contrary, on numerous occasions he declares people to be Apikorsim, including frum Jews , as per #7 above. What he did say is that the Halchacha of Moridin v’Lo Maalin – that you are allowed to kill an Apikores – does not apply nowadays, because we are not able to fulfill the prerequisite of properly giving them tochachah before hand, as well as the lack of open miracles nowadays.
9) Even though usually we have a rule that we try to be melamed zechus (judge favorably) on Jews, where an Apikores is concerned, the contrary is true. We are not melamed zechus at all. On the contrary, someone who refuses to curse the Apikorsim, is halachicly suspect to be an Apikores himself.
And here
There is a Halachah that says you must have the right Hashkofos. “Lo sosuru acharei levavchem”, You may not follow your heart, meaning, you may not believe “meenus” (Apikursos). Th Mishna Brura rules (his source is Sefer Hachinuch almost word for word, but without attribution) Apikursos includes any opinion that is contrary to Daas Torah.
Meaning, even if you follow all Halachos, if you have an opinion that conflicts with that of the Torah, you violate this laav.
Halachic example: Responsa Divrei Chaim YD 105. The case was a rebbi in a cheder who expressed his opinion to his class that the commentary “Ohr Hachaim” on chumash was a great commentary, but it was not written with Ruach HaKodesh. This statement created a tremendous controversy, and they turned to the Divrei Chaim for a ruling.
After explaining that he cannot rule on a specific incident without hearing both sides of the story, he writes that theoretically, if someone says such a thing, since it is clear that Chazal disagree, he would be guilty of Apikursus.
So let’s say you’re a vegetarian. There is no Halachah that says you have to eat meat (at least not on the weekdays). But if the reason you don’t eat meat is because you believe shechitah is cruelty to animals and therefore wrong, you are guilty of Apikursos, since the Torah clearly disagrees.
Two people can do the same act – here, refraining from eating meat – but one is a spiritual criminal and the other innocent, because of the attitude with which the action was taken.
Or let’s say you are in favor of women’s torah education. The Chofetz Chaim was, too, for our times. But if you feel that women’s education is an advancement for women’s rights, essentially an improvement in the treatment of women over what we have been accustomed to in the past generations, when we did not teach Torah to women – and the Chofetz Chaim did not feel that way – you are guilty of Apikursus, since your belief collides with that of the Torah’s.
If someone accepts ideas that are not in accordance with the Torah, in other words, Hashkofos, they violate this issurThere are different levels of Apikursos. Worst-case scenario, someone can become a full-fledged Apikores, which Halachicly is considered worse than in idol worshipper. Such a person is treated Halachicly like a non-Jew, yet retains the halachic obligations of all Jews. A full-fledged Apikores is the absolute bottom on the spiritual food chain.