Q: Is it permissible to defame a companion of Bida' or to speak against him and criticize him?
A: The people of Bida' are of two kinds:
Either he conceals it to himself - or he is well known amongst the people. As for the first instance then there is no need to make it well know, because his misguidance is limited to his own self.
As for the other type then it is necessary to defame him and warn against him, so that the people who live amongst him are not deceived by him; And this is not backbiting at all - like some of the obstinate ones presume - and the hadeeth Of Backbiting: 'you mentioning about your brother that which he dislikes to be mentioned' this hadeeth is from those that are general and which is specific.
I have mentioned to you the saying of some of the Scholars in two lines of poetry, which gathers the type of backbiting which is exempted from being Haram, so they say:
القدح ليس بغيبةٍ في ستةٍ .....متظلم ومعرِّفٍ ومحذرٍ ومجاهر فِسقاً ومستفتٍ ومـن..... طلب الإعانة في إزالة منكر
Defamation is not backbiting regarding six types of people**
The oppressed and for recognition and the one warned against***
The one who openly sins and the one who necessitates a verdict***
The one who seeks aid in removing an evil***
Mutathalumin : being oppressed - Mua'arafin: recognition - Muhadeerin: Warning against Mujahir fisqan : One who openly sins - Mustaftin : Inquiring a fatwaa
Istee'ana fee Ezzalatin Munkarin : seeking assistance in changing the evil
So defaming the Mubtadi' enters into this definition and it also enters into warning against him; this is why the Scholars of hadeeth - May Allaah reward them with goodness - are agreed upon characterizing many of the narrators of hadeeth with what they had innovated into the Deen. The scholars of hadeeth mention all of this due to them standing up with the obligation of clarifying it for the people; So that it would be known who is the strong narrator and his 'Aqeedah is accepted. The narrator who is a Mubtadi' is abandoned along with his deviated 'Aqeedah.