You are a communist
“You are a communist,” is an attack occasionally directed at an atheist by a theist. This claim is an example of an ad hominem, wherein one attacks the personal character of his or her debate partner, instead of attacking the arguments presented. Therefore, even if the atheist in question is a communist, it has relatively little bearing on the questions of theism/atheism. In the US, this association is largely the result of cold war propaganda that contrasted the "free, capitalist West" with "atheistic, communist Russia". 
- "Are Communist Atheists & are Atheists Communists? Are there any differences between both groups are are they both the same thing? "
There is no evidence supporting the claim that most atheists are communists. An atheist can simply ask the offending theist to define the term “communist”, and then point out ways in which he or she differs from the communist socialist movement. A primary point to consider is that atheism and theism are metaphysical philosophies, whereas communism is a political ideology; communism cannot be demonstrated to follow necessarily from atheism. Unlike communists, most atheists do not seek to ban religion but they do often support secularism.
If, for the sake of argument, we accept that most communists are atheists, it does not follow that most atheists are communists. This is the logical fallacy known as “affirming the consequent.”
Communists are atheists?
The argument references the fact that communist regimes are often nominally atheist. For instance, China has introduced religious tests to ensure their power is not undermined by organised religion. However, this does not show that all or most atheists are communists.
A similar argument is to make the accusation that Stalin was an atheist, which is of questionable accuracy.
Various communist writers have stressed that atheism is a necessary part of communism. However, it is questionable if the population generally shares this view. Chinese culture has been strongly influenced by Chinese folk religion, Confucianism, Taoism and Buddhism. There are also tens of millions of Christians in China. Most people in Soviet Russia were Orthodox Christians.
Most communists are atheists vs. most atheists are communists
The argument proceeds as follows:
- If P, then Q.
- Therefore P.
In this case:
- If someone is a communist, then he or she is likely an atheist.
- I am an atheist.
- Therefore I am likely a communist.
This argument is invalid because statement three can be (and often is) false, even granting as true statements one and two. We could consider another example that should make it clear to the theist that the argument holds no weight:
- If someone refuses blood transfusions on religious grounds, he or she is likely a theist.
- I am a theist.
- Therefore, I am likely to refuse blood transfusions on religious grounds.
If a theist makes the claim, “You are a communist,” and will not subsequently concede the invalidity of the claim, discussions with him or her are unlikely to be fruitful. You may wish to question whether further discussions with this person on the topics of atheism and theism are necessary or productive.