Do communists believe or not believe God?

Generally speaking, communists are atheists because communists believe in what is called a "Material History", as described by Karl Marx. This version of history sees material wealth as the central driver of history. So a rough depiction of what history has been in the eyes of a communist would be this:

1. Slavery (small number of people reap wealth with a large number of people owned and doing most of the work; an example would be ancient Greece, where captured enemy soldiers were slaves)

2. Feudalism (nobles own the land, and serfs are tied to the land; the serfs would have to give a certain percentage of their income to the nobility)

3. Capitalism (a small number of indivduals own the "means of production" - factories, investment capital, etc - while a large number are employed by them for the least amount possible)

This view of history led Marx to believe that at each stage the "underclass" was being relied upon to do more and more complex work and would have to become more and more educated. Eventually they would realize they are being subjugated and would revolt, leading to the "Worker's Paradise" (Marx's family owned many factories in Germany and the conditions of the workers left a profound impact on Marx).

Because this version of history seems divorced from a history in which God intervenes on Man's behalf, and because Marx viewed religion as oftentimes being used by elites to keep the people from realising the reality of their condition, Marx called religion the "opiate of the masses" and most communists agree with him.

Of course, as with any ideology, it is always different based upon the time and the place so it is difficult to say all communists are atheists. Christian Socialism is a form of soft communism which is not atheistic. But, in generaly, Marxist ideology embraces Atheism.

Additionally, a true Marxist embraces Dialectical Materialism. The human mind is not "soul driven" . The ideas of humans are based on Marx's belief that the brain secretes thought. This is contrary to the belief in a Creator.