I believe that not even liberals know which was the origin of their own thought nor which was/is the real goal of it .. I am going to explain it the shortest as possible.
To start, pure marxism (19th century) promised that a change on the infrastruture (conditions of production) would necessarily bring a change on the superstructure (mind/moral/culture). So it promised that once Govt directed production through a central plan, people would start to love socialism, and behave like real socialists, in short, a new man.
Then later, the socialist revolution happened in Russia (1917), and they applied pure marxism: Govt removes property and starts to direct production through a central planning. However that did not bring that change of minds/morals/culture that marxism promised, to the contrary, the Soviet Union had to used huge amounts of violent coercion to keep people acting like they liked socialism.
Around the 1920's and 1930's mainly, some marxists from the West, visited the Soviet Union, and realised that that way of bringing socialism did not work, the mind of people had not changed and then Govt had to smash people with terror to acept socialism.
Then those marxists thought that marxism did not bring the real goal of socialism (a new man) because people had into their minds a moral/culture that radically opposed to socialism: christianity. And then those marxists (named since then liberals) proposed to do the contrary process than pure marxism. They believed that inside the capitalist west a socialist revolution of the soviet sort was impossible, specially cause capitalism far from making people poorer was making people richer along time. They decided to change the superstructure (mind/thought/culture) first and that would bring that people would love a socialist central planning of production (infrastructure).
And that is what they have been doing since then, specially from after WW2, and obviously from the 60's, with that "cultural terrorism" (they themselves called it that way) ...
to be continued ...