Any authoritarian government is going to have some core similarities to religions, especially to monotheism, due to it's inherent authoritarian power structure (one unquestionable, unelected authority). Neither can maintain their existence and control if people are free to think for themselves and act in their interests, or even the majority long term interests. Thus, they must use a blend of emotional manipulation, coercion, and force. Both will promote the virtue of obedience and blind loyalty, punished with violent threats. The facts will sometimes not favor what is in the interests of the authority. Both will seek to limit liberty of action and of thought (and thus attack "intellectuals"), b/c maintaining their control will require denial of at least some facts. Similarly, both will demand that loyalists ultimately just "have faith" in the authority (either "the state" or "god/the church"), even when their own rational mind says the authority is wrong. Both will tend to demean and promote bigotry against those not loyal to their authority, which includes towards other religions. Their need for blind allegiance makes allegiance to anything else a threat. Communism is confrontational to religion, but only 1 more religion than most religions are, which is all religions but itself.
Note that Marx himself advocated open critique of religion (not violence) to move people rationally away from religion, b/c it's authoritarian and controls people against their own interests. He was correct. The more aggressive and sometimes violent attack of religion by the Communist state is a byproduct of the fact that communism in a large industrial society cannot possibly be maintained without aggressive authoritarian control. That made organized religious authorities a direct threat competing for mindless loyalty. It had nothing to do with anything inherent to atheism. They could have done what most other authoritarian governments do, which is to take control of the dominant religion and eliminate others. The communist rulers' goal was to preserve their own power, but since they were pretending to follow Marx's philosophy and to care about the interests of the people, they couldn't support any one religion over others.
