The faults of those who slander the dharma are extremely grave.
In The King of Concentrations Sutra, it states that the sins of slandering and abandoning the sutras, and teachings, are extremely weighty. As it is equivalent to one having destroyed all stupas in Jambudvipa, and killing arhats that are as many as grains of sand in the Ganges River.
If we are able to arise the 3 greatness*, we will realize that not a word by the Buddha can be abandoned. It is to be fully viewed as the instructions for true practice, and to generate a respectful attitude. In this way, we will not differentiate if the teachings are good or bad. We will naturally prevent serious defamation of the dharma.
Translated from the excerpts from Liberation in the Palm of Your Hand
*3 greatness - Outline of text for Lamrim Chap 2 The Greatness of the Teaching:
II. Showing the greatness of the teaching in order to engender respect for the instructions
A. The greatness of enabling one to know that all of the teachings are free of contradiction (pg 46)
B. The greatness of enabling one to understand that all of the scriptures are instructions for practice (pg 50)
C. The greatness of enabling one easily to find the Conqueror's intent (pg 53)