Many religions hate to be mocked. In fact, there is a term for disrespecting religious people, ideas, and fetishes: Blasphemy.
Islam has led the way in sensitivity in the last few years but it is by no means the only religion that wants extra "respeck" on their name.
Sometimes these insults are direct and intentional. Other times the insults are unintentional or the result of a misunderstanding. There is no doubt that unbelievers have prodded the theists on that front. But rarely has the disrespect that believers levy on each other has been examined. I will discuss a few ways that I've seen many religious people be disrespectful to each other with little reprimand.
The Little g
Because of the popularity of monotheism and how the English language mechanics work, you will often see the capitalization of the word "God" used for the preferred diety of the person writing something. I've seen and read several examples of believers deliberately using a lowercase "g" to write about other deities that they don't worship. Sometimes these gods are defunct. Other times it is used to talk about gods that are currently en vogue. That is where the disrespect is overt. The offender often claims that their god is the "true" god while the others are somehow imposters. Other times the capital "G" is used to imply a uniqueness to the preferred deity while talking about other gods worshiped by others.
For those gods that dare take a physical form, there are devotees who worship non-physical gods who have been directed to hate them. People who have undetectable gods often call people who have physical gods "Idolaters". This is meant to be a derogatory term. They NEVER use it in a positive way. A few years ago, some Muslims in Afghanistan destroyed some ancient statues of Buddha. Members of ISIS have filmed themselves destroying ancient artifacts that date back to ancient Sumer and Mesopotamia. These acts were fueled by the ingrained disdain that the Abrahamic gods have for physical competitors.
There are hundreds of religions. Within those religions, there are thousands of denominations. The Abrahamic Trio of Judaism, Christianity, and Islam have forged a divide between themselves and everyone else. I've never heard the members of the Trio use the term "Pagan" without contempt. Growing up I thought that a Pagan was a depraved person. I had no idea that it was the Hebrews who were the savages as reported by their holy book. The term tends to trend heavily during the fall and winter holidays when conservative types try to out-Pharisee each other by showing disinterested people that their jolly holidays have "Pagan" roots.
In my opinion, the naming of the Hebrew Bible as an "Old Testament" is a slight to the Jews. Christians believe that a new covenant was made with them by the Hebrew god Yahweh when Jesus became the focus of worship for the new cult. Just like that, all the promises and binding commitments of Yahweh shifted to his new darling followers. Of course, the Christians don't see it that way. They see their bible (the New Testament) as PART of the Jewish canon and as one book (Protestants, Catholics, and Eastern Orthodox bibles have 66, 73, and 81 books respectively).
Jews do not see the New Testament as divinely inspired at all. About 600 years after the supposed death and resurrection of Jesus came the last revelation to mankind via Muhammad. So in a way, the New Testament (or Gospels/Injeel) is now a compromised and antiquated document; at least to Muslims. Needless to say, Christians do not acknowledge Muhammad just like the Jews do not acknowledge Jesus. Human blood has been spilled over these concepts in order to prove which tribe the god of Abraham favors at the moment. Who knows, maybe the canon will continue to evolve.