“Religion is a species of mental disease. It has always had a pathological reaction on mankind.”
As quoted by Mussolini in 2000 Years of Disbelief: Famous People with the Courage to Doubt by James A. Haught (1966) p. 256. From a speech he made in Lausanne, July 1904.
1900s
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Benito Mussolini 127
Duce and President of the Council of Ministers of Italy. Le… 1883–1945Related quotes

as quoted in "Keynes and the Ethics of Capitalism" by Robert Skidelsy http://www.webcitation.org/query?id=1256603608595872&url=www.geocities.com/monedem/keyn.html
Essays in Persuasion (1931), Economic Possibilities for our Grandchildren (1930)
Context: When the accumulation of wealth is no longer of high social importance, there will be great changes in the code of morals. We shall be able to rid ourselves of many of the pseudo-moral principles which have hag-ridden us for two hundred years, by which we have exalted some of the most distasteful of human qualities into the position of the highest virtues. We shall be able to afford to dare to assess the money-motive at its true value. The love of money as a possession — as distinguished from the love of money as a means to the enjoyments and realities of life — will be recognised for what it is, a somewhat disgusting morbidity, one of those semi-criminal, semi-pathological propensities which one hands over with a shudder to the specialists in mental disease … But beware! The time for all this is not yet. For at least another hundred years we must pretend to ourselves and to everyone that fair is foul and foul is fair; for foul is useful and fair is not. Avarice and usury and precaution must be our gods for a little longer still. For only they can lead us out of the tunnel of economic necessity into daylight.

Patheos, Anti-theist Answers to Christian Questions http://www.patheos.com/blogs/reasonadvocates/2015/11/22/anti-theist-answers-to-christian-questions/ (November 22, 2015)
[Morris Kline, Mathematics: The Loss of Certainty, http://books.google.com/books?id=RNwnUL33epsC&pg=PA203, 1982, Oxford University Press, 978-0-19-503085-3, 203]

“The mentality of mankind and the language of mankind created each other.”
Modes of Thought (1938).
1930s
Context: The mentality of mankind and the language of mankind created each other. If we like to assume the rise of language as a given fact, then it is not going too far to say that the souls of men are the gift from language to mankind. The account of the sixth day should be written: He gave them speech, and they became souls.

“Nationalism is an infantile disease. It is the measles of mankind.”
When asked by Viereck if he considered himself to be a German or a Jew. A version with slightly different wording is quoted in Einstein: His Life and Universe http://books.google.com/books?id=dJMpQagbz_gC&lpg=PP1&pg=PA386#v=onepage&q&f=false by Walter Isaacson (2007), p. 386
1920s, Viereck interview (1929)
Variant: Nationalism is an infantile disease. It is the measles of mankind.
Context: It is quite possible to be both. I look upon myself as a man. Nationalism is an infantile disease. It is the measles of mankind.

“Religion is a psychic disease of the brain.”
1900s, God Does Not Exist (1904)

“Religion…is a man's total reaction upon life.”
Lecture II, "Circumscription of the Topic"
1900s, The Varieties of Religious Experience (1902)