What is the ruling on someone who insults the Christian religion?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If what you meant by the Christian religion, the religion that was revealed on the Prophet ‘Eesaa (Jesus) then this is not permissible, rather this is an act that takes a person out of the fold of Islam. It is not permissible to insult any religion among the religions that were revealed to previous Prophets, may Allaah exalt their mention; all the religions of the previous Prophets were one. Indeed the Muslims believe in all the Prophets. Allaah Says (what means): {The Messenger has believed in what was revealed to him from his Lord and [so have] the believers. All of them have believed in Allaah and His angels and His books and His messengers, [saying], “We make no distinction between any of His messengers.”}[Quran 2:285]. Moreover, insulting any religion which is revealed to the Prophets, may Allaah exalt their mention, or mocking it, is an act that takes a person out of the fold of Islam. Allaah Says (what means): {Say, “Is it Allaah and His verses and His Messenger that you were mocking?” Make no excuse; you have disbelieved [i.e. rejected faith] after your belief.}[Quran 9:65-66]. In another verse Allaah Says (what means): {He has ordained for you of religion what He enjoined upon Nooh (Noah) and that which We have revealed to you, [O Muhammad ], and what We enjoined upon Ibraaheem (Abraham) and Moosaa (Moses) and ‘Eesa (Jesus) [may Allaah exalt their mention], to establish the religion and not be divided therein.}[Quran 42:13]. However, if what you meant by religion, what the Christians have innovated and falsified, and other acts by which they associate partners to Allaah, then there is no harm in insulting it and describing the Christians as disbelievers and non-Muslims. Indeed, Allaah Says (what means): {They have certainly disbelieved who say, “Allaah is the third of three.}[Quran 5:73]. Nonetheless, if doing this would lead other people to insult Islam or the Prophet of Islam then it is not permissible to do so, as a way of preventing evil from taking place. Indeed, Allaah forbade us from insulting the idols of the disbelievers so that this would not lead to the idol worshippers insulting Allaah. Allaah Says (what means): {And do not insult those who invoke other than Allaah, lest they insult Allaah in enmity without knowledge.}[Quran 6:108].

Allaah Knows best.