Joseph,
“Nebech an apikorus is ocheit an apikorus” was the shita of Rav Chaim Brisker. Not everyone holds that way. Rav Chaim was coming to answer why the Rambam lists only 13 ikkarim (and other Rishonim list fewer). Don’t we have to believe everything in the Torah, how can their only be 13?
The answer, according to Rav Chaim, is that these 13 (according to the Rambam) are the beliefs that define yehadus. If you don’t believe these 13, you don’t get a chelek in the next world, you are not a Torah Jew. Not as a punishment, but simply because you haven’t met the necessary basic criteria. Ma sh’ein kein, everything else in the Torah, we are required to believe. But if someone has a mistaken belief about them (say he believes the world was created in 8 days), he is not an apikorus.