Marxists really don't want you to check out how hard he went in on Marxism...I hate that his quote is often used solely against Nazis. Karl Popper hated Marxists as well, and often criticized Karl Marx's writings. He also worked alongside the likes of Friedrich Hayek and Milton Friedman, whom many lefttists despise with a passion.
So great was his annoyance at the theories of Marx, Freud and Adler being treated like they had the authority of science that this is a preamble to Popper going full autism and nailing down exactly what it means for something to be scientific.I found that those of my friends who were admirers of Marx, Freud, and Adler, were impressed by a number of points common to these theories, and especially by their apparent explanatory power. These theories appeared to be able to explain practically everything that happened within the fields to which they referred. The study of any of them seemed to have the effect of an intellectual conversion or revelation, opening your eyes to a new truth hidden from those not yet initiated. Once your eyes were thus opened you saw confirming instances everywhere: the world was full of verifications of the theory.
Whatever happened always confirmed it. Thus its truth appeared manifest; and unbelievers were clearly people who did not want to see the manifest truth; who refused to see it, either because it was against their class interest, or because of their repressions which were still ‘un-analysed’ and crying aloud for treatment.
It's a crying shame Soros appropriated the name of Popper's book for his Marxist revolutionary funding platform and that the commies will of course try to make it out that Popper wouldn't be applying the exact quotes about "tolerance of intolerance" against them right now if he were currently alive, but that's how the rules for radicals bunch roll.