The core thesis of Breivik's manifesto is William Lind's "cultural Marxism" conspiracy theory, and the terms "cultural Marxism" or "cultural Marxist" appear over 600 times in the manifesto. Analyst and researcher Chip Berlet explains Breivik's thesis, from Lind, with the following formula:

"Cultural Marxism=Political Correctness=Multiculturalism=Muslim Immigration=Destruction of Judeo-Christian nations"

Anders Breivik was so gripped by William S. Lind's "cultural Marxism" conspiracy theory concerning the origin of "political correctness" that Breivik's manifesto plagiarized, with minor modifications and additions by Breivik, the entire body of William Lind's 2004 Free Congress Foundation book "Political Correctness:" A Short History of an Ideology.

From pages 11 to 37, Breivik's manifesto reproduces the core of the Free Congress Foundation book (pages 4-51) edited by William S. Lind - whose introductory chapter by Lind, "What is Political Correctness", Breivik lifted almost unchanged.

Breivik's manifesto also borrowed, on page 13, the exact words of William S. Lind from Lind's introduction to a 1990s twenty-two minute Free Congress Foundation video, on the origins of "political correctness":

"Just what is "Political Correctness?" Political Correctness is in fact cultural Marxism (Cultural Communism) - Marxism translated from economic into cultural terms."

That Free Congress Foundation video also featured Laszlo Pasztor, a close working colleague of FCF founder Paul Weyrich. During World War Two, Pasztor was "a liaison between the Hungarian Nazi party and Berlin" and after the war served a five year prison sentence for crimes against humanity.

In 1988 Pasztor was at the center of a national scandal over the involvement of Eastern European emigres who had collaborated with the Nazis, but had been allowed into the U.S. because of their fierce anti-communist beliefs, in the presidential campaign of George H.W. Bush.

The Lind and Weyrich book The Next Conservatism explicitly and repeatedly references the cultural Marxism theory that influenced Anders Breivik and, very early in the book, explains it in the most dire and cosmic terms:

'Americans' most fundamental freedoms, including freedom of speech and freedom of association, are under threat from the ideology most commonly known as "multiculturalism" or "political correctness". But what really is "PC"? A tour through a bit of esoteric intellectual history reveals its secret: it is cultural Marxism, Marxism translated from economic into cultural terms by a think tank established in 1923 in Frankfurt, Germany, the infamous Institute for Social Research. Cultural Marxism's goal from the outset has been nothing less than the destruction of Western culture and the Christian religion, goals toward which it has made frightening progress. The next conservatism must arm Americans against this menace with the weapon it fears most: the revelation of its real nature.' (p. 5, The Next Conservatism)

On page 39 following a several page explanation, in detail, of the "cultural Marxism" conspiracy theory, The Next Conservatism refers readers to the URL of a free book on the website of the Free Congress Foundation, "Political Correctness: A Short History of an Ideology" - the very 2004 book plagiarized in Anders Behring Breivik's political manifesto.

Following Anders Behring Breivik's 2011 terrorist attack, that 2004 Free Congress Foundation book disappeared from the Free Congress Foundation's website.