“For an idea that does not first seem insane, there is no hope.”

Speech to the Classical Association (8 January 1926), quoted in On England, and Other Addresses (1926), p. 107.
1926
Context: Believing as I do that much of the civilisation and culture of the world is bound up with the life of Western Europe, it is good for us to remember that we Western Europeans have been in historical times members together of a great Empire, and that we share in common, though in differing degrees, language, law, and tradition. That there should be wars between nations who learned their first lessons in citizenship from the same mother seems to me fratricidal insanity.

Source: A Man of Law's Tale (1952), At the Scottish bar, p. 39

Source: The Night Eternal

Google shows that the internet often attributes this statement to Einstein, but never with a source. It does not occur in any book in Google Books.
Misattributed

Source: Different Seasons

from "The Pasternak Affair"
Disputed Questions (1960)