Hilchos Lashon Hara chapter 8, section 4
It is lashon hara to speak even about someone ignorant of Torah, for he is part of G-d’s nation taken from Egypt. How much more if he is learned in which case the sin is far worse. The sages (Berachos 19a) says, “Whoever speaks after the death of a Torah learned person, falls into gehinom as it says, etc.” Often, this puts one in the category of someone who despised a Torah learned person. The great punishment of such a person is mentioned in Sanhedrin (99b). (Also, the Yoreh De’ah 243:6 rules that he counts among those of whom it said, He despised the word of the L-rd, etc., that soul shall surely be cut off.) However, the evil inclination persuades a person that despising a Torah learned person only applied in the time of the Talmud when they were very wise and not in our times. This is an absolute error because the status of a Torah learned person is determined according to his generation even in our time. If he is fit to give halacha ruling and toils in Torah, he is called a talmid chacham and if someone disparages him, even only verbally, and even in his absence, committed a serious sin and is culpable for banning as we find in Yoreh De’ah (243:7 and 334 in the Shach paragraph 68. If the talmid chacham issues halacha rulings in the town the transgression is far worse for besides the obligation to treat him as a talmid chacham and honor him since he relies on his rulings, furthermore, by disparaging him one removes people from G-d’s service, for through this people will say, “Why should we go to him for our monetary disagreements if he is not fit to decide who is right?” Through this, each person will make his own decisions. (Many other detrimental things result from this that are too many to mention).