“A lie told often enough becomes the truth.”

Source: Under the Glacier

Other

The Near East (1968), p. 31
General sources

It (1986)

Thriller (1982)

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

From Terrorism & Tyranny: Trampling Freedom, Justice and Peace to Rid the World of Evil (Palgrave, 2003) http://www.jimbovard.com/Epigrams%20page%20Terrorism%20&%20Tyranny.htm

Source: East of Eden