In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Whosever Does Not Declare a Mubtadi as a Mubtadi is a Mubtadi Himself

Al-Allaamah Rabee [may Allaah preserve him] said:

It is not correct to apply this principle unrestrictedly because it maybe that he is unaware of the Bidah committed by that person, therefore he refrains from declaring him a Mubtadi [an innovator] due to [ورع – i.e. fear of falling into a doubtful matter that might lead him to something forbidden and affect him in the afterlife]. Why should you declare him an innovator? [i.e. why should you declare him an innovator when it is the case that he is unaware of the bidah of that Mubtadi or refrains from making Tabdee out of ورع ]. However, if he knows the innovator [i.e. knows that such a person is upon bidah due to which the evidence was established against him], loves him and allies with him, then he is a Mubtadi. This is the distinction in this affair. He knows that that person is a Mubtadi but he aids him and wages war against Ahlus Sunnah Wal Jamaa’ah, then he is a Mubtadi without a doubt. As for a person who does not know that such and such person is a Mubtadi, he is not to be declared an innovator.

You [i.e. the scholar] – the one who studied the affair and you know that he allies with a Mubtadi, places him in a favourable position, wages war against Ahlus Sunnah for the sake of that Mubtadi and for the sake of this falsehood, then this person is a Mubtadi, misguided. As for the person who does not know that indeed such and such person is a Mubtadi, then advise him and clarify for him [i.e. with clear unambiguous proofs until he understands] that such person is a Mubtadi. So, unless he ceases [i.e. refrains from defending this Mubtadi after understanding the evidences], then put him (in the same place) with his companion – the Mubtadi. (1) Read more here about this affair: https://abukhadeejah.com/shaikh-rabee-establishment-of-proof-in-declaring-a-person-to-be-an-innovator/

Imaam Al-Awzaa’ee [may Allaah have mercy upon him] said, “O Muslims! Fear Allaah, accept the advice of the sincere advisers and the admonition of the admonishers. know that this knowledge is religion, so beware of what you do, from whom you take your religion, the one you follow and the one with whom your religion is safe, because indeed all the proponents of innovation in religious affairs are all falsifiers, liars and sinners. They are not cautious – neither do they pay attention nor do they fear, and addition they are not to be trusted that they would not distort what you hear.

They say what they do not know with regards to what they disapprove and when proclaiming their lies, but Allaah (knows, hears and sees) what they do. Therefore, be cautious of them, trust them not, reject them and avoid them, because indeed this is what your earlier scholars did and commanded, and likewise the righteous ones who came thereafter.

Be careful of opposing Allaah and being the means to destroying the religion which He has ordained – breaking its commands and prohibitions, nullifying and weakening it by respecting the innovators in religious affairs and those who initiate that which is not from the religion, because you know what has been transmitted (i.e. the prohibitions) about respecting or exalting them. And what can be greater respect and exaltation than acquiring your religion from them- following and believing them, drawing close to them and assisting them in enticing the ones they entice and woo to their views and the religion (i.e. religious innovations) they follow. This is sufficient to be regarded as associates of theirs in what they do. (2)

[Ref1: Taareekh Dimashq 6/361]
[Ref 2: An Excerpt from ‘Awnul Baari 2/891’. slightly paraphrased]