Is criticizing the opinions of others – perhaps extending to criticizing the person and their creed in their absence – considered backbiting (gheebah) and slander (nameemah), and what are the consequences thereof?

Backbiting is mentioning a Muslim with what he dislikes concerning his faults, and it is unanimously forbidden by any means whatsoever. As for criticizing individuals with the intention of deterring them from their bad deeds, or warning against their innovations, or exposing their falsehood which they have openly declared, this is permissible and not forbidden backbiting. Rather, it may be an act of devotion or an obligation, because what matters is the intention and purpose.