Are communists atheists?

Answer

Some are, some are not. This question cannot be answered in a more complete way. It has no common context. It would be like asking, are janitors atheists, or are grocery clerks atheists? Or, even, what does blue taste like? There is no commonality for either concept beyond their mutual existence in the world.

Communism is an economic (and political) perspective. Atheism deals directly with theology or "atheology" as the case may be. While Leninist/Stalinist/Maoist communism typically precludes religious organizations from legal existence, it is more for political purposes than theological ones. Churches have power over the people. The two major communistic governments of the world sought to remove power from all but the state. Hence the eradication of religious groups. Keep in mind however that the Greek Orthodox church continued to exist in some form during the entire reign of Soviet Communism, the Roman Catholic church enjoyed a certain degree of freedom during Czechoslovakia's communist era (now two separate countries Czech Republic and Slovakia, with parliamentary democracies, Roman Catholics make up the largest religious group in both countries), and although it was repressed so does Buddhism in China.