Never engage someone in a discussion of someone else's thought, even if he believes in it wholeheartedly.
1. He may be misunderstanding or misrepresenting what the author/text is saying. An example: a discussion last week of how AM defines modernity.
2. In a dialogue, one is concerned not with what some accepted authority teaches, but whether that teaching is true or not. So if the other person says, "I believe that x's teaching on y is true," the reply is to see whether this teaching has sufficient support or not, not whether he is reporting the teaching correctly because presumably he believes in this teaching, regardless of whether it is actually that of the author or not.
If it turns out that he's just a yes-man, then a different approach must be taken.
3. This principle is even more important if you yourself are not familiar with the texts or authors to which he refers. So a common basis for dialogue must be sought elsewhere--our knowledge of reality, which should be the primarily resource anyway.