How many people in Communist Russia and China have been killed because of atheism and secularism?
How can that be? After all, millions and millions of people died in Russia and China under communist governments and those governments were both secular and atheistic, right? So weren't all of those people killed because of atheism indeed, in the name of atheism and secularism?
No, that conclusion does not follow. Atheism itself isn't a principle, cause, philosophy, or belief system which people fight, die, or kill for. Being killed by an atheist is no more being killed in the name of atheism than being killed by a tall person is being killed in the name of tallness.
People were killed in communist nations for a lot of different reasons. Some were communists who disagreed with those in power and were killed because of that. Some were anti-communists opposed the government and were killed for that. Some were simply in the way or inconvenient and were killed for that. These are political disagreements that people were being killed over, not murder in the name of atheism.
But weren't a lot of people killed because they were Christian? Certainly but not simply because they were Christian. Communists typically regarded religious organizations as a hinderance towards the creation of a worker's paradise. Some religious groups also opposed the communists. Once again, we are generally looking at political issues, not a question of atheism.
Even if some people were killed simply because they followed a religion, it does not follow that they were killed in the name of atheism. Why? Because atheism is not inherently opposed to religion: it is possible to be both an atheist and religious and some religions are themselves atheistic. Atheism also isn't a belief system or ideology which can, by itself, inspire people to do things good or bad.
To understand this better, consider times in the past when religion has been involved with violence the Inquisition would be good. How many people were killed during the Inquisition in the name of theism? None. Those doing the killing acted not because of theism, but rather because of Christian doctrines. The belief system is what inspired people to act (sometimes for good, sometimes for ill). The single belief of theism, however, did not.
Similarly, communism certainly inspired people to act and gave them motivations to do certain things, but atheism which is the absence of a belief and not even a belief itself did not. The assumption that people in Russia and China were killed merely on account of atheism is based upon two other myths: first, that atheism is itself some sort of philosophy or belief system which can motivate people, and second that atheism is somehow interchangeable with the actual belief system of communism. It also pretends that all the various elements of communist totalitarianism were irrelevant to what happened which is utter nonsense.
The aforementioned parallel explains why this response is not one which religious theists can use to deny their religion's responsibility for violence in the past. Atheism and theism may not themselves be sufficient to justify violence and murder (or good behavior, for that matter), but belief systems which incorporate them are more than sufficient. Communism (or at least certain forms of it) can be blamed for communist violence; Christianity (or at least certain forms of it) can also be blamed for Christian violence. As a belief system with specific doctrines that were openly held up as justifying or sanctioning violence, religion must be held responsible for the violence committed in its name.
Whether theism can be slightly more culpable than atheism is a matter of dispute. Not being any belief at all, atheism can't motivate anyone in any direction to do anything. Theism is a belief, however, so at least the potential for some sort of motivation in some direction exists. It's been argued, for example, that monotheism is inherently more prone to violence because of the way it tends to be exclusivist unlike polytheism, which tends to be more tolerant of cultural and religious differences.
It's difficult to say, though, how many of these problems are really inherent in the type of theism and how many are cultural products of the religious belief systems that incorporate them. Whatever culpability theism itself might have, it's likely small enough to dismiss, allowing us to treat it and atheism as functionally equal in this context.