"Not all atheists are communists", on that point you are definitely correct, but ALL Communists are by definition atheists, Marx was a proponent of Hegelianism and a "material atheist"
Karl Marx said "Religion is the opium of the people". also stating: "Communism begins from the outset with atheism; but atheism is at first far from being communism; indeed, that atheism is still mostly an abstraction."
Lenin wrote: "A Marxist must be a materialist, i. e., an enemy of religion, but a dialectical materialist, i. e., one who treats the struggle against religion not in an abstract way, not on the basis of remote, purely theoretical, never varying preaching, but in a concrete way, on the basis of the class struggle which is going on in practice and is educating the masses more and better than anything else could."
Engels wrote of atheistic evolutionism and Communism: "Just as Darwin discovered the law of development of organic nature, so Marx discovered law of development of human history."
Chinese Communist leader Zhou Enlai declared, "We Communists are atheists".
In 2014, the Communist Party of China reaffirmed that "members of the party must be atheists."
In 2016 “A senior Chinese advisor on religious affairs has said the country should promote atheism throughout society,"
Hell Cambridge university contends that the Russian Revolution was the most notable point in human history, that contributed to the spread of Atheism.
Try to remember that some peoples facts are Truths, while other peoples "Truths", are not facts at all!
