“Anything is true if enough people believe it is.”

Source: Addicted Magazine - https://www.weraddicted.com/matkai-burmaster-wants-us-to-be-fearless-about-our-streaming-and-digital-content-creating/

Source: Invisible Monsters

Kōnosuke Matsushita (1989) Nurturing Dreams My Path in Life. Quoted in: Tony Kippenberger (2002), Leadership Styles: Leading 08.04. p. 73

Source: Wizard's First Rule

This quotation actually comes from page 211 of Émile Cammaerts' book The Laughing Prophet : The Seven Virtues and G. K. Chesterton (1937) in which he quotes Chesterton as having Father Brown say, in "The Oracle of the Dog" (1923): "It's the first effect of not believing in God that you lose your common sense." Cammaerts then interposes his own analysis between further quotes from Father Brown: "'It's drowning all your old rationalism and scepticism, it's coming in like a sea; and the name of it is superstition.' The first effect of not believing in God is to believe in anything: 'And a dog is an omen and a cat is a mystery.'" Note that the remark about believing in anything is outside the quotation marks — it is Cammaerts. The correct attribution was reportedly first traced by Pasquale Accardo. http://www.chesterton.org/ceases-to-worship/ It was also credited to Nigel Rees (as cited in First Things, 1997). http://books.google.com/books?id=NuQnAAAAYAAJ&q=%22The+first+effect+of+not+believing+in+God+is+to+believe+in+anything%22&dq=%22The+first+effect+of+not+believing+in+God+is+to+believe+in+anything%22&hl=en&ei=PSzcTvewIefx0gHqmrj0DQ&sa=X&oi=book_result&ct=result&resnum=2&ved=0CDUQ6AEwAQ
Misattributed

Original: Per essere immediatamente scomodi alla maggior parte delle persone, è sufficiente essere veri, corretti e sinceri.
Source: prevale.net

"Intense Ornate" interview with Amazon.co.uk (1999) http://www.elizabethhand.com/interview99.shtml
Context: So much fantasy relies on the author's having read Fraser's The Golden Bough or Robert Graves' The White Goddess and nothing else. The White Goddess is a crank book, a crank book of genius of course, but all the same... Mind you, I found Waking the Moon cited in an article in a pagan magazine as an authority for the idea that there was a patriarchal brotherhood, the Benandanti, that have been running things since antiquity, with no mention of the fact that it is a novel, and a fantasy at that. People want to believe something, and so they swallow anything.

Compare sourced quote set forth above: "The English follow the principle that when one lies, it should be a big lie, and one should stick to it."
Attributed to Goebbels in Publications Relating to Various Aspects of Communism http://books.google.com/books?id=iLAnAQAAMAAJ&q=%22If+you+repeat+a+lie+often+enough,+people+will+believe+it.%22&dq=%22If+you+repeat+a+lie+often+enough,+people+will+believe+it.%22&hl=en&sa=X&ei=U4gPUvObG4qMyQHlhYAw&ved=0CGQQ6AEwCQ (1946), by United States Congress, House Committee on Un-American Activities. No reliable source has been located, and this is probably simply a further variation of the Big Lie idea.
Variants:
If you repeat a lie often enough, people will believe it.
If you repeat a lie often enough, it becomes the truth.
If you tell a lie big enough and keep repeating it, people will eventually come to believe it.
If you repeat a lie long enough, it becomes truth.
If you repeat a lie many times, people are bound to start believing it.
Attributed in The Sack of Rome (2006) by Alexander Stille, p. 14, and also attributed in A World Without Walls: Freedom, Development, Free Trade and Global Governance (2003) by Mike Moore, p. 63.
Misattributed

Source: Bleak Seasons (1996), Chapter 77 (p. 212)