Bismillah,
Warning against the deviant and straying people is an obligatory matter. In fact, if warning against the person who cheats the Muslims in articles of trade is obligatory, then it is more so to warn against the person who intrigues and perverts the Religion and belies Allah and the Messenger.
Allah, ta^ala, said in Surat Al-^Imran, Ayah 104: which means: [Let there be among you a nation that invite to the good, command the lawful, and forbid the unlawful.]
Abu ^Aliyy ad-Daqqaq said: "The one who withholds the truth is a mute devil."