It is easy for Protestants as they consider only the Holy Scripture to be sacred. Catholicism and Orthodoxy consider the Sacred Tradition a necessary part of the religion and that the very Church is the Christ's body. It is an absolutely fundamental norm and thus violating the (particular) Church is an act of blasphemy.
"Fundamentalists" today mainly means people who deny the tradition and rely on the original sacred texts only, that is come to fundamentals. That's what al-Wahhab have done in Islam and he created islamic fundamentalism. In such a definition all Protestants are Christian fundamentalists.