so Rabbanim have the right to call other Rabbanim apikursim.
The Rambam says 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. (The Raavad holds that he is not, however, he agrees that even though the person himself is not considered an Apikores, his mistaken belief is indeed considered Apikorsus.)