What is the ruling on a Muslim reviling religion and on reviling the religion of the one who reviled it, given that the former commits an act of disbelief while the latter does not, because the latter reviled the religion of a disbeliever?

Reviling a religion which causes its utterer to commit an act of disbelief (kufr) is the revilement of the true religion of a Muslim. It is not the revilement of a false religion like Zoroastrianism. If the second person reviles the religion of the first, then either his intention is to revile the disbelief into which the first person fell because of his revilement of the religion, in which case he does not commit an act of disbelief by that; or his intention is to revile the religion of the one being reviled, with his knowledge that it is Islam, in which case both are equal in the revilement.