The basic idea of Torah scholars that are demons [Le"M vol I chapter 12]. [The Gemara itself brings the idea that demons can be knowledgeable in Torah and also that they can possess a person. Therefore the logical deduction is that a person possessed by a demon that is knowledgeable in Torah will be knowledgeable in Torah. [The note on the bottom of the page in the Le.M brings the source of Rav Nahman from the Zohar, but I do not see why since Rav Nachman also had a source in the Gemara itself.
[It is brought in expanded form in the Ari also. ] This seems to me to be one of Rav Nahman's most important ideas. It provides a warning to people that could be too easily taken in.
Is there any test, to know the difference between good and evil in this regard? There is, but I can not tell what it is exactly.
However I did want to add a comment. First that David Bronson did point out to me that even Rav Yaakov Emden did hold that some parts of the Zohar are authentic. (Large portions of it were added to.)
Furthermore there are plenty of warnings about religious leaders in the Talmud and the Prophets also.
