This was what might be called 'cultural Marxism'. It was based on the understanding that what holds a society together are the pillars of its culture: the structures and institutions of education, family, law, media and religion. Transform the principles that these embody and you can thus destroy the society they have shaped. [This is basically what I was taught in grad school. In turn, I taught it to my students and graded their work on the degree to which they were willing to support a leftist cultural revolution. We dressed it in liberal democratic terms, but what we pushed and pushed and pushed was cultural Marxism. The family is a particularly vile institution to the leftist b/c the family is the most immediate threat to the power of the state.]
[Antonio] Gramsci [Italian Marxist philosopher] understood that the working class would never rise up to seize the levers of 'production, distribution and exchange' as communism had prophesied. Economics was not the path to revolution. [No, it's not. Humans beings were created to perfect their natures against the model of the Word Incarnate. We are not here merely to survive but proposer! Leftists consistently prop up gov't programs that help people survive. Not a bad thing in itself, of course, but how often do these programs trap people in mere survival? One of Aquinas' many insights was that even the Virtuous Pagan can embrace the Good in the context of striving for perfection by living a virtuous life. You don't need Christ to be a good person. . .but you do need him to be a Perfected Person.]
He believed instead that society could be overthrown if the values underpinning it could be turned into their antithesis: if its core principles were replaced by those of groups who were considered to be outsiders or who actively transgressed the moral codes of that society. [Thus B.O.'s appointment of Bizarre Czars with radical backgrounds. It's all about diversity and tokenism and never about competence or talent.]
So he advocated a 'long march through the institutions' to capture the citadels of the culture and turn them into a collective fifth column, undermining from within and turning all the core values of society upside-down and inside-out.
HT to Lynn