One of the many beliefs -- i.e., non-empirically based doctrines -- of the post-Christian West has been that moral progress is the human norm, especially so with the demise of religion. In a secular world, the self-described enlightened thinking goes, superstition is replaced by reason, and reason leads to the moral good.
Of course, it turned out that the post-Christian West produced considerably more evil than the Christian world had. No mass cruelty in the name of Christianity approximated the vastness of the cruelty unleashed by secular doctrines and regimes in the post-Christian world. The argument against religion that more people have been killed in the name of religion than by any other doctrine is false propaganda on behalf of secularism and Leftism.
The amount of evil done by Christians -- against, for example, "heretics" and Jews -- in both the Western and Eastern branches of Christianity -- was extensive, as was the failure of most European Christians to see Nazism for the evil that it was. The good news is that Christian evils have been acknowledged and addressed by most Christian leaders and thinkers.
But there were never any Christian Auschwitzes -- i.e., systematic genocides of every man, woman and child of a particular race or religion. Nor were there Christian Gulags -- the shipping of millions of innocents to conditions so horrific that prolonged suffering leading to death was the almost -inevitable end.
The anti-religious Left offers two responses to these facts: The first is that modern technology made the Nazi and Communist murders of scores of millions possible; had the church been technologically able to do so, it would have made its own Auschwitz and Gulag. The second is that Nazism and Communism were religions and not secular doctrines.
The response to the first is that technology was not necessary for the Communist murders of over a hundred million innocent people in the Soviet Union, China, Cambodia and elsewhere. In Cambodia, millions were murdered with hammers, in Rwanda with machetes.
The response to the second is that Communism and Nazism were secular movements and to deny that is to tell a gargantuan lie. Even if one argues that Nazism and Communism were religions, they were nevertheless secular religions. That too many Christians morally failed when confronted with Nazism is true, but irrelevant to the fact that Nazism was in no way a Christian movement.