sacred cow 
noun
An idea, custom, or institution held to be above criticism (with reference to the Hindus' respect for the cow as a holy animal): the sacred cow of monetarism the British diet remains a sacred cow...
- It has consistently tackled the sacred cows of sex, religion and politics with wit, irony and hard criticism leaving behind carcasses of exposed folly and evil.
- The idea of paying bloggers is a controversial one, as it challenges some of the sacred cows of the journalistic publishing business.
- We need constructive criticism of our most sacred cows.