You have to read the Communists' playbook.
Karl Marx Communist Manifesto: "When the ancient world was in its last throes, the ancient religions were overcome by Christianity. When Christian ideas succumbed in the 18th century to rationalist ideas, feudal society fought its death battle with the then revolutionary bourgeoisie. The ideas of religious liberty and freedom of conscience merely gave expression to the sway of free competition within the domain of knowledge.
“Undoubtedly,” it will be said, “religious, moral, philosophical, and juridical ideas have been modified in the course of historical development. But religion, morality, philosophy, political science, and law, constantly survived this change.”
“There are, besides, eternal truths, such as Freedom, Justice, etc., that are common to all states of society. But Communism abolishes eternal truths, it abolishes all religion, and all morality, instead of constituting them on a new basis; it therefore acts in contradiction to all past historical experience.”"
Karl Marx
Private Property and Communism
"This
material, immediately
perceptible private property is the material perceptible expression of
estranged human life. Its movement – production and consumption – is the
perceptible revelation of the movement of all production until now, i.e., the realisation or the reality of man. Religion, family, state, law, morality, science, art, etc., are only
particular modes of production, and fall under its general law. The positive transcendence of
private property as the appropriation of
human life, is therefore the positive transcendence of all estrangement – that is to say, the return of man from religion, family, state, etc., to his
human, i.e.,
social, existence. Religious estrangement as such occurs only in the realm of
consciousness, of man’s inner life, but economic estrangement is that of
real life; its transcendence therefore embraces both aspects. It is evident that the
initial stage of the movement amongst the various peoples depends on whether the true
recognised life of the people manifests itself more in consciousness or in the external world – is more ideal or real. Communism begins from the outset (
Owen) with atheism; but atheism is at first far from being
communism; indeed, that atheism is still mostly an abstraction."
Vladimir Lenin discusses war on religion
The Attitude of the Workers’ Party to Religion
https://www.marxists.org/archive/lenin/works/1909/may/13.htm
Same with Josef Stalin
http://hollowverse.com/joseph-stalin/
