Sam2: Emunas chachamim is mentioned in avos perek 6 as one of the 48 things required in order to aquire torah.
The fact that a human being can reach the level of ruach hakodesh is part of torah sheba’al pe. The written sources are many. (See braisa of Rabi Pinchas Ben Yair which forms the basis of the sefer Mesillas Yeshorim, for example.) One who denies this reality is a kofer in an area of torah sheba’al pe and is therefore an apikorus. See Rambam Hilchos Rotzeach 4:10 uncensored version for what should be done to kofrim and apikorsim.