If you show up to an animal rights rally with a bucket full of fried chicken for lunch, you may be accused of committing sacrilege. You are violating a belief held sacred, at least by some individuals.
Sacrilege has its roots in the Latin sacr-, meaning "holy." Sacrilege was originally reserved for talking about blasphemous acts that disrespect, violate, or misuse holy traditions or objects. However, today the term sacrilege carries a broader, and lighter, meaning than its origins suggest. For example, it's usually considered sacrilege to root for the out-of-town team that's playing against your home team. Ancient users of this word might think our looser definition is sacrilege.