What is the ruling on insulting a person's religion, assuming he is Christian, then it becomes clear that he is Muslim?

Originally, it is permissible to curse the distorted religion of the Christians. However, it may be prohibited if it leads to cursing our religion or our Prophet, based on the Almighty's saying: "And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge."

If a person curses another's religion, thinking it is Christian, and then it becomes clear that the person is Muslim, he does not commit an act of disbelief by this cursing. This is because he did not intend to curse Islam, but rather intended to curse disbelief. And Allah Almighty says: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended."