I have never heard anyone other than Rangers fans called huns.
As a Protestant myself, I have never been called a hun in my life. It would actually be quite interesting if I was to be arrested and charged with a religious hate crime for using the word hun at a Rangers game (I use the word freely during games against them). How would the court deal with me effectively being accused of a hate crime against my own faith?
The word hun is used exclusively for fans of Rangers and no-one else. Don't let them pretend otherwise.