Is it obligatory for a Muslim to revile other polytheistic and distorted religions and their beliefs, or is there no blame in refraining from reviling them?

As long as a Muslim believes in the invalidity and disbelief of the Christian religion, he is not obligated to insult anyone or their religion. As for the permissibility of insulting, it is elaborated as follows: If the intended meaning is the religion brought by Jesus, peace be upon him, then it is impermissible to insult it, and doing so is disbelief, for Muslims believe in all prophets and their scriptures. However, if the intended meaning is the distortion, alteration, and polytheism introduced by the Christians, then there is no objection to insulting it and describing its adherents as polytheists and disbelievers, as long as this does not lead to insulting the religion of Islam.