Is it permissible to backbite or revile an unbeliever?
Ans. That is permissible since there is no
respect/inviolability for an unbeliever, unless that will
cause some problem. For example, if in reviling him to
his face or behind his back he finds out, then he may
revile the Muslims and his Deen and the Messenger (S) in
which case it is forbidden.
