krapp gave a decent summary, but I think if you look closer, you find that Qanon is a variety of factions lumped under a single label. The one unifying thread of belief is near religious devotion to Trump. Trump's minions might do wrong (Jeff Sessions is an early example of a Q-favorite that later was found wanting), but Trump himself is beyond any flaw.
QAnon is a far-right conspiracy theory. It alleges that a cabal of Satan-worshiping pedophiles is running a global child sex-trafficking ring and plotting against President Donald Trump, who is battling against the cabal. The theory also commonly asserts that Trump is planning a day of reckoning known as "The Storm", when thousands of members of the cabal will be arrested. No part of the theory is based on fact.
Although preceded by similar viral conspiracy theories such as Pizzagate, the theory proper began with an October 2017 post on the anonymous imageboard 4chan by "Q", who was presumably a single American individual. It is now likely 'Q' has become a group of people. Q claimed to have access to classified information involving the Trump administration and its opponents in the United States. NBC News found that three people took the original Q post and expanded it across multiple media platforms to build internet followings for profit. QAnon was preceded by several similar anonymous 4chan posters, such as FBIAnon, HLIAnon (High-Level Insider), CIAAnon, and WH Insider Anon.
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QAnon may best be understood as an example of what historian Richard Hofstadter called "The Paranoid Style in American Politics", the title of his 1964 essay on religious millenarianism and apocalypticism. QAnon's vocabulary echoes Christian tropes—"The Storm" (the Genesis flood narrative or Judgement Day) and "The Great Awakening"—evoking the reputed historical religious Great Awakenings of the early 18th century to the late 20th century. According to one QAnon video, the battle between Trump and "the cabal" is of "biblical proportions", a "fight for earth, of good versus evil." Some QAnon supporters say the forthcoming reckoning will be a "reverse rapture": not only the end of the world as we know it, but a new beginning, with salvation and utopia on earth for the survivors.
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The Washington Post and The Forward magazine have called QAnon's targeting of Jewish figures like George Soros and the Rothschilds "striking anti-Semitic elements" and "garden-variety nonsense with racist and anti-Semitic undertones". A Jewish Telegraphic Agency article in August 2018 asserted: "some of QAnon's archetypical elements—including secret elites and kidnapped children, among others—are reflective of historical and ongoing anti-Semitic conspiracy theories".
The Anti-Defamation League reported that while "the vast majority of QAnon-inspired conspiracy theories have nothing to do with anti-Semitism", "an impressionistic review" of QAnon tweets about Israel, Jews, Zionists, the Rothschilds, and Soros "revealed some troubling examples" of antisemitism.
The Czarist hoax The Protocols of the Elders of Zion has intersected with the QAnon conspiracy theories, with Republican QAnon fan Mary Ann Mendoza retweeting a Twitter thread about the Rothschild family, Satanic High Priestesses, and American presidents saying that "The Protocols Of The Elders Of Zion Is Not A Fabrication. And, It Certainly Is Not Anti-Semitic To Point Out This Fact." Mendoza, who sits on the advisory board of Women for Trump and was scheduled to speak at the 2020 Republican convention until news came out of her Twitter activity, later denied knowing the content of the thread, although anti-Semitic references appeared in the first few tweets. Similarly, Trump has denied knowing anything about QAnon except that QAnon fans like him and "love our country."
By 2020, QAnon followers were advancing a theory that Hollywood elites were engaging in "adrenochrome harvesting," in which adrenaline is extracted from children's blood to be oxidized into the psychoactive drug adrenochrome. Adrenochrome harvesting is rooted in antisemitic myths of blood libel dating to the Middle Ages that Jews murder Christian children for their blood for use in religious rituals. QAnon believers have also promoted a centuries-old antisemitic trope about an international banking conspiracy orchestrated by the Rothschild family.
Genocide scholar Gregory Stanton has described QAnon as a "Nazi group rebranded", and its theories as a rebranded version of the Protocols of the Elders of Zion.
Few other cases like that escaped the social networks and became a direct danger to real people. Few got politicians to display signs they agree with the conspiracy ideas on public TV.
I guess FB tries not to land in the headlines about someone going postal after organising the attack in semi-public on their site.
Most of what QAnon believes is what the American right wing and many Conservative/Evangelical Christians already believed - particularly in linking secular society/leftism/"globalist elites" with pedophilia and conspiracies of satanism. None of that is new, some of it is very old indeed. In my day we just referred to it as the "New World Order."
What's new is how QAnon packaged and rebranded these disparate bigotries into a single narrative with Donald Trump as the messiah and made it viral, likely on the back of the general alt-right and neo-reactionary surge of support within Trump's base. But it wouldn't have taken off at all if that wellspring of hate in America's core hadn't been there to tap, and there weren't already plenty of people who would have found the general premise of, say, Hillary Clinton drinking the blood of child sex slaves, to be reasonable to begin with. You don't convince people to believe that sort of thing who aren't already halfway there.
I would strongly dispute that "many" Evangelical Christians believe that secular society, leftism, and global elites are linked to either pedophilia or satanism.
Nor was that the idea of the "New World Order". Globalist elites? Sure. Secularism and leftism? Yes. Conspiracy? Absolutely. Pedophilia and satanism? No.
Rumors about satanic rituals came up everywhere from Denver International Airport to Skull and Bones and Bohemian Grove to the Bilderberg Group... at least wherever NWO conspiracies overlapped with the Evangelical demographic that took things like Left Behind and John Hagee seriously. The satanic panic happened in the 1980s.
Maybe "many" is an overreach, but those beliefs were definitely present well before Pizzagate and Qanon.
I would bet that an event like Jeffrey Epstein being suicided in federal prison under impossibly suspicious circumstances -- after it came out that he had been running a child sex trafficking ring of immense scope, been granted a sweetheart deal during his first legal run-in in 2007, and had close ties to the uppermost echelons of wealth & power including Bill Clinton, Donald Trump, and Bill Gates -- has probably also greatly expanded the number of people willing to take the theory seriously. Alan Dershowitz even says he was introduced to Ghislaine Maxwell by Lynne de Rothschild herself!
The short version is that there is a global ring of powerful pedophiles that Trump is the only one capable of stopping who also happen to be anyone the far right hates... and also jews.