The mid-19th c was phenomenal. So many brilliant composers, scientists, philosophers, genuine geniuses like Marx and Darwin. The age of Prometheus. What Man could achieve seemed limitless. Man shaking his fist at God. Or even taking over, revolting against God. Atheism became the fashion among the intelligentsia. It's as if God got His revenge in the 20th c, showing Man just how childish, ignorant, hateful he could be. Without God. So was it all Marx's fault? What would he do?
Next to workers of the world unite!, Marx's religion is the opium of the people is a meme that is embedded in our consciousness. But like most good things in life, it is misquoted.