Catholics regard using the consecrated host for any purpose besides Communion as sacrilegious. If you burn a copy of the Koran, Muslims find it sacrilegious. Sacrilegious means very disrespectful towards something sacred.
The Latin sacrilegus meaning "thief of sacred things" was used to describe robbers who plundered graves and temples. You can consider something sacrilegious to be as disrespectful as a graverobber towards things that others find holy. In modern ironic usage, any opinion that runs contrary to popular wisdom can be called sacrilegious. If you call the movie, "Citizen Kane" boring and self-indulgent, movie lovers might think you sound pretty sacrilegious.
