"Nothing will benefit human health and increase chances for survival of life on Earth as much as the evolution to a vegetarian diet."
"It is my view that the vegetarian manner of living by its purely physical effect on the human temperament would most beneficially influence the lot of mankind."1
"Vegetarian food leaves a deep impression on our nature. If the whole world adopts vegetarianism, it can change the destiny of humankind."2
As much as recreational carnivores might wish that Albert Einstein's status as a vegetarian was as much a myth as Hitler's, it is true that he supported the idea3. This is perhaps why "Einstein Was A Vegetarian: Think About It" is a more popular T-shirt slogan than "Hitler Was A Vegetarian: Think About It", especially amongst the crusading college student crowd.
However, an argument from authority in favour of vegetarianism is not strong: Einstein was a physicist, not a dietician, statistician, psychologist, nor oracle. In fact, he is a more likely authority in debates on the existence of God, as metaphysics is only one order away from physics.
1: According to the International Vegetarian Union (http://www.ivu.org/history/northam20a/einstein.html)
this is from a letter to the Vegetarian Watch-Tower on 1930-12-27.
2: From an online book called "Vegetarianism Vs. Non-vegetarianism"
(http://www.jinvani.com/eng/bookshelf/Veg%20vs.%20NonVeg10.htm)
3: According to the International Vegetarian Union (http://www.ivu.org/history/northam20a/einstein.html), there is no evidence that Einstein actually practiced vegetarianism.