Marx admired the philosopher Hegel. Hegel believed that history was a series of conflicts wherein a "thesis" and an "antithesis" collided. As a result of the collision, both the thesis and the antithesis morphed into something new, which Hegel called the "synthesis". This new synthesis would be prevailing paradigm until it collided with an antithesis, and the cycle would play out all over again. Marx adopted Hegel's idea, and said that all of history was progressing steadily upwards towards communism.
Later thinkers applied Marx's ideas to other aspects of society besides economics. Thinkers in Frankfurt, Germany known as the Frankfurt School" used Marxism to come up with what they hoped would be a theory of all reality. This new theory was based on the concept of oppressor/oppressed classes. It divided individuals into groups by race, class, or gender, and was critial of historical western civilization. Critical Theory was born.
Cultural Marxists look at all aspects of life through the lens of the oppressor/oppressed class dynamic. This philosophy found widespread acceptance among post-modern thinkers. Marx's philosophy called for a violent revolution to overthrow the oppressor class and usher in the Dictatorship of the Proletariat. Depending on the thinker, Cultural Marxism may or may not contain the element of violent revolution.
Marx and the Frankfurt School were critial of religion in general, and Christianity in particular. This is partially because the Christian worldview, with it's concept of original sin, denies that man is inherently good. This means that the world is not naturally progressing upwards toward communist utopia for, as Jesus says, "The poor you will have with you always." Moreover, in the cultural Marxist system people are not looked at as individuals, but as groups. This leads to the concepts of collective guilt and collective salvation. No matter how "good" a member of the oppressor class is, they are still guilty of the evils of society because they belong to the oppressor class. This is directly opposed to Christianity, which teaches that men are redeemed from sin and death by the death and resurrection of Christ; they are called as individuals to repent, and believe the Gospel.
T.R. Halvorson at Steadfast Lutherans wrote a good article that dives a little more deeply into the Frankfurt School called, "Cultural Marxism and Critical Theory: An Introductory Sketch." Take a moment to check it out.
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Cooper, Jordan B. July 28, 2018 "Cultural Marxism Explained."
Dr. Jordan B Cooper. YouTube. http://youtube.com/watch?v=rj1fjy0iM6U. Accessed on 29-May-2021.