"The term 'sacred cow' has come to mean any stubborn loyalty to a long-standing institution which impedes natural process." It comes from India's treatment of the cow as sacred animal. 
        The pure and simple truth is rarely pure and never simple. Oscar Wilde
        
Westerners often imagine that if these poor ignorant people would only slaughter the cows their diet would improve and starvation would end. In fact the cow is an intricate and uniquely essential part of the agricultural economy, ecology and existence of life in India. If the cow was not religiously considered "sacred" it would have been wiped out many years ago during times of famine and a greater social and cultural disaster would have followed. 
        Truth is sacred and if you tell the truth too often nobody will believe it. G. K. Chesterton 
         Even sacred cows can serve a purpose and their removal should not be done lightly or casually. Often the stubborn, mindless, religious loyalty to an idea or belief is unreasonable, inconvenient and even detrimental, still the alternative often remains even more frightening, disastrous and unthinkable. 
        It is well for people who think to change their minds occasionally in order to   keep them clean. For those who do not think, it is best at least to rearrange   their prejudices once in a while. Luther Burbank 
        There are many   sacred cows in the lives of men. They are found in religion, government, science   and philosophy. Often the more educated a man, as related to "degrees" of   knowledge and diplomas, the more stubbornly he seems to cling to his personal   sacred cows, even in the face of facts and reason. The more a man takes "pride"   in what he thinks he knows and believes the more likely it is that he is   harboring sacred cows. Still when reason and wisdom are in short supply it is   often the sacred cows and superstitions that keep men in unbridled destruction. 
        The ultimate result of shielding men from the effects of folly   is to fill the world with fools. Herbert Spencer
         Religion is   probably the single most abundant source of sacred cows. It has been one of the   most important and stabilizing influences on the order of mankind, individually   and as a society, while at the same time it has been one of the most volatile   fuels for the inflammation of bloody wars and agonizing inquisitions. That is   also why we will wisely roast our first sacred cow taken from the barn of   governmental philosophies. 
        I never give them hell. I just tell the truth   and they think it's hell. Harry Truman
                    
          Roasting a sacred cow is a   dangerous business and something is likely to get burned in the process. Our   interest is not in the death or injury to the cow or its owner. Our interest is   not so much in roasting the cow but in scrubbing it clean and grooming it a bit   so that everyone can see what the cow's actual condition, value and purpose is. 
        I can't understand why people are frightened of new ideas. I'm   frightened of the old Ones. John Cage
        Maybe a better metaphor is not so   much that we are roasting a sacred cow but that we are just baptizing the   congregation and cleaning out the barn. The cow is just getting hosed down a bit   in the process. I am sure some may think we are out to kill the cow but I assure   you that is not our intention.
        Only reason can convince us of those   three fundamental truths without a recognition of which there can be no   effective liberty: that what we believe is not necessarily true; that what we   like is not necessarily good; and that all questions are open. Clive Bell