That's question #1106 from R. Zev Leff's web site, which someone called to my attention. Interestingly enough, the category it is placed into is 'Halacha (General Jewish Law).'
Hear the answer. If not, here is my transcription:
"Is a person who does not believe that the Zohar was revealed to Shimon bar Yochai counted as a heretic--Dovid from Beit Shemesh--the authenticity of the Zohar has been accepted by Klal Yisrael, and therefore one who does not believe in the Zohar as being a part of Torah she-be-'al peh is, yes, a heretic."
In trying to understand his point of view, it occurred to me that the question was formulated a specific way and the answer restates the question another way, and then a third way.
The question uses this term:
"the Zohar was revealed to Shimon bar Yochai"
R. Leff then speaks of "the authenticity of the Zohar" and, finally, "the Zohar as being a part of Torah she-be-'al peh."
I'm not sure what to make of these, if there are distinctions, intended or not. Also interesting is that its authenticity being accepted by Klal Yisrael appears to be the deal maker here.