Chazal say that it is forbidden to learn even Torah from one deficient in emunah, lest one be influenced by their ways.
(שבת עה ע"א, שו"ע יו"ד סי' קל)
It once happened that a certain talmid chacham conducted himself so inappropriately that Rav Yehuda wanted to place him in cherem. However, he was hesitant to do so, for then the local talmidim would not be allowed to learn Torah from him.
Rav Yehuda therefore consulted with Rabba bar Bar-Chana, who told him: The possuk says that one should learn Torah from a talmid chacham “since he is a malach of HaShem.” This teaches us that only if the talmid chacham conducts himself like a malach should one learn Torah from him. Since this talmid chacham is acting as he does, Torah may not be learnt from him. Nothing will therefore be lost by placing him in cherem.
(מו"ק יז ע"א, שו"ע אדה"ז הל' ת"ת פ"ד הי"ז)
The Rebbe adds another requirement: one should seek to learn Torah from a teacher who is sensitive to its kedusha, to its connection with HaShem. Whatever the listener learns from such a teacher will then impact his neshama fruitfully.
(שיחו"ק תש"כ ע' 195)
