Is it permissible to mock other religions, such as saying that the fasting of the people of the Christian faith is nonsense?

The religions existing today, other than Islam, have been corrupted. However, it is not permissible to mock any act of worship mentioned in their holy books, because it is possible that it has not been corrupted, in which case the mocker would have mocked what Allah has legislated. And even if it was invented by their rabbis, it should not be mocked to prevent the possibility of mocking the laws of Islam.