to be a “kofer” means to reject something. So, one needs to have access to the thing as a perquisite to accepting or rejecting it. In our conversation about being kofer in Torah, that would mean that one would need access to that part of Torah. Rejection (and so too acceptance) can happen either generally or specifically, explicitly or implicitly. So, one can be mekabel on himself the complete Torah despite not knowing or having access to the whole body of Torah, and so too rejecting the Torah.
This is also true for any details. If one DOES have access to them, he can accept or reject those details in kind. In Torah, the 13 ikarim include accepting the complete written and oral Torah, such that claiming that even one word was taught by Moshe – chalila – alone and not Hashem would be literally kofer (v.) in that Torah.
There is a very interesting deep dive into the minutiae of debate amongst Rishonim about what the guidelines of “access to” are in “edge cases”. In other words, what is the expectation for a person to accept the 13 ikarim generally or specific details if they don’t have formal access to the information. This touches on the sugya of tinuk shenishbu as well as the Brisker Rav’s statement that “nebuch an apikorusis still an apikorus”.
It is well established to the point of being obvious that there are parts of Torah that are unknown to our generation, haluchas that have been lost as well as new questions that haven’t been resolved through the tools we have at hand (mishna and shikil daas). The ultimate answer to these unknowns will be revealed by Eliyahi Hanuvi in the future. In the meantime there is no expectation for us to somehow be certain in knowing the answers to these parts of Torah that we can’t possibly have access to.
Nonetheless, we are obligated to be certain (not have any even fleeting doubt – as is true for all 13 ikarim) that the Torah that we DO have today is exactly the true Torah that was given by Hashem at Sinai. The parts that we don’t know, we are obligated to accept and look forward to the time when we will be able to learn those parts.
If a person knowingly rejects, either by way of a general or a specific, any part of the Torah, that person would be a kofer (n.). If he acknowleges that he doesn’t (yet) know some detail, but accepts it as much as possible, he is not a kofer.
Mistakes
Mistakes are easy to understand in the same manner. If a person is intentionally making a mistake – such as rejecting accepted mesora of what was taught at Sinai or not learning that mesorah to his ability – he would be guilty of rejecting the true Torah. On the other hand, if he has no access to the true Torah, he would not be guilty of rejecting anything.
To summerize: a “mistake” in Torah is by definition a false “non-Torah” idea and that would make it by definition “kefira”(n.) vis-a-vis the true Torah. If someone would adopt that mistake he would be “kofer”(v.) in the Torah. And if he did this purposfully, he would be a “kofer”(n.). (and someone who themselves is a “kofer” in the Torah, would be a “kofer b’ikar” of the “ikar” relevant to believing the Torah we have today is from Sinai)
Machlokes Hillel Shamai
In order to delve into kefira vis-a-vis Hillel and Shamai, you would need lengthy introductions about other core concepts of “lo bashomayim hee” and “klalei psak and horaah”. If that is interesting, we can get into it, lmk.
For the sake of this conversation, it is enough to know that debates between Gedolim on the level of Hillel and Shamai were ones of “shikil daas” not mistakes in of forgeting – chalila – mishne. The process of oral Torah is designed to enable a process of new situations to establish longstanding right and wrong of “shikil daas”. Once that process of establishment happens, that “shikil daas” enters the body of oral Torah that must be excepted by future generations.
Before (or during) that “shikil daas”. a person who is “roy l’horaah” could argue on either side bettween those competing “deyas” and he would not be kofer in Torah.
In conclusion, on the level of Hillel and Shamai, since the debate was specifically only in terms of undecided “shilik daas”, there would be no grounds for accusations of kefira towards either side.