Shaykh ul-Islaam Ibn Taymeeyah -Rahimullaah- said:

‘And if giving advice is obligatory for general and specific religious interests: like regarding narrators of hadeeth who make mistakes and lie then like Yahya bin Sa’eed
said: I asked Malik, Thawri, al-Laith bin Sa’ad – I think it was him - and al-Awza’ee about a man who is accused in hadeeth or he does not memorize hadeeth? They said: Make clear his condition.’

Some of the people said to Ahmad bin Hanbal: ‘I find it difficult to say about a person that he has such and such problem, and so and so is like such and such.’
Imam Ahmad said: ‘If you remain silent and I remain silent when will an ignorant person know the authentic from the unauthentic?!’
Likewise the leaders of Bida’ from the people who have statements that oppose the Book and the Sunnah, or worship which opposes the Book and the Sunnah, it is obligatory to clarify their condition and warn the Ummah against them and this is in agreement with the Muslims.

So much so that it was said to Ahmad bin Hanbal: Is a person who fasts, prays and makes Itikaf more beloved to you or speaking against the people of Bida’?’

He answered: ‘If he fasts, prays and makes Itikaf then that is for himself, and if he speaks against the people of Bida’ then that is for the Muslims and that is better.’
So it is clear that this benefit is general for all the Muslims for their religion and it is in the same class as Jihad in the path of Allaah. Since purifying the path of Allaah and His Deen, His Minahij (methodologies), His Sharia’ and repelling the injustice of these people and their hostility upon this is a general responsibility, which is something that is in agreement among the Muslims.

If it was not the case that Allaah made people in order to repel the harm of these people, the Deen would have become corrupted. The corruption of the Deen would be greater than the corruption caused by being conquered by the enemy at war. Since if these people take over they do not corrupt the hearts and what is in the heart is from the Deen, but they do it slowly, as for the People of Bida’ then they corrupt the hearts from the onset.’

[Taken from ‘Majmoo fatawa’ 28/231-232]