“I know I'm not God, are you? Don't be silly.
God? God? Everybody's God? Don't be silly.”

Source: It Happened One Autumn

Source: The Tales of Alvin Maker, Red Prophet (1988), Chapter 4.

Source: Interview with Piergiorgio Odifreddi in Incontri con menti straordinarie (TEA, Milano, 2007), ISBN 978-88-502-1523-2.

Response after being asked "Do you regard yourself as a religious person?", in an interview with Religion & Ethics Newsweekly http://www.pbs.org/wnet/religionandethics/2011/02/18/february-18-2011-ernest-gaines/8169/, February 18, 2011

Reported in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 86. The quotation has been attributed to many others, such as Lord Chief Justice Campbell, Lord Chesterfield, Sir William Harcourt, Lord Pembroke, Lord Westbury, and to an anonymous judge, and said to have been spoken in court to Garter King at Arms, Rouge Dragon Pursuivant, or some other high-ranking herald, who had confused a "bend" with a "bar" or had demanded fees to which he was not entitled. George Bernard Shaw quotes it in Pygmalion (1912) in the form, "The silly people dont [sic] know their own silly business."

Maule cannot be the original source of the quotation, as it is quoted nearly twenty years before his birth in Charles Jenner's The Placid Man: Or, The Memoirs of Sir Charles Beville (1770): "Sir Harry Clayton ... was perhaps far better qualified to have written a Peerage of England than Garter King at Arms, or Rouge Dragon, or any of those parti-coloured officers of the court of honor, who, as a great man complained on a late solemnity, are but too often so silly as not to know their own silly business." "Old Lord Pembroke" (Henry Herbert, 9th Earl of Pembroke) is said by Horace Walpole (in a letter of May 28, 1774 to the Rev. William Cole) to have directed the quip, "Thou silly fellow! Thou dost not know thy own silly business," at John Anstis, Garter King at Arms (though in his 1833 edition of Walpole's letters to Sir Horace Mann, George Agar-Ellis, 1st Baron Dover, attributes the saying to Lord Chesterfield in a footnote, in the form "You foolish man, you do not understand your own foolish business"). Edmund Burke also quotes it ("'Silly man, that dost not know thy own silly trade!' was once well said: but the trade here is not silly.") in a "Speech in the Impeachment of Warren Hastings, Esq." on May 7, 1789 (when Maule was just over a year old). Chesterfield or Pembroke fit best in point of time.
Attributed

Scotland in the World Forum (February 4, 2008), Church of Scotland (May 25, 2009)

Source: Love in the Time of Cholera

Catch-22 (1961)