“Patur Aval Assur, regarding your question how someone who holds like R’Hillel be an apikoires: If there is a machlokes in the Gemorah if one is allowed to do something on Shabbos and we pasken le’chumrah. If someone goes and does it nowdays, he is a mechallel Shabbos. The same is with the R’Hillel’s shittah.”
That’s because a psak can be set and it can be changed but it makes no difference what the reality is. When it comes to beliefs, the things that we are believing are either true or not true. And that can’t change from the time of the Gemara to now. Beliefs are taluy on the metzius. Now if you want to tell me to believe something even though it might not be true i.e. we pasken what you have to believe, see my response to DaasYochid in which I point out the cognitive dissonance involved in such a position.