A meme is a collection of cultural ideas that can be transmitted between minds via various forms of communication and that may respond to selective pressures. We can think of memes as cultural analogues of genes, which are similarly affected by natural selection. Many atheists find memes to be useful constructs for understanding religious beliefs.
One particularly influential meme that has been propagated by many American Christians is the claim that Hitler was an atheist and that atheism played an important role in the atrocities he committed. While this meme is by no means universally accepted among Christians (e.g., many modern White supremacist groups idolize Hitler and are proud of their Christianity), it is common enough that every American atheist will encounter it sooner or later. From what I can tell, the function of this particular meme is less about history and more about distancing oneself from the bad behavior of one's Christian ancestors while impugning the morality of modern atheists.
Austin Cline has written an excellent post about the inaccuracy of this meme, showing how it is largely a product of the "no true Christian" fallacy (i.e., the Christian version of the "no true Scotsman" fallacy). He also refers his readers to what sounds like a fascinating read, The Holy Reich: Nazi Conceptions of Christianity, 1919-1945. I'll be adding this one to my ever expanding reading list.
When we encounter the "but Hitler was an atheist" claim, we should recognize it as being part of a larger anti-atheist meme. The Christian making such a claim is willing to distort reality, ignore evidence, and make false statements in order to protect what must be a fragile belief system. At the same time, we must never become too comfortable that those with reality on their sides will always triumph. It may take some work.