But in any case, our beliefs are very important too. If we are not allowed to believe something, it is possible that it is kefira (at least on some level) to believe it.
If there was something in Chazal that was defiantly disproven without a shadow of a doubt, It cannot be Kefira to belive it. If Chazal ever said it cannot ever get dark in the middle of the day and you experienced a Solar Eclipse, If you belived the eclipse you just experienced actually happend, according to you, you are a Koifer