"Whoever is a Communist is not a Muslim. Communism involves different kinds of blatant kufr, such as denying the existence of Allaah (may He be glorified and exalted), denying the existence of the unseen worlds, making a mockery of the religion of Allaah and looking down on it, and flouting its laws and morals. How can the Muslim whose religion is Islam follow an ideology which states that religion is the opium of the people? -
islamqa.info/en/ref/3321 "