Hitler and the Nazis are often cited as an example of the horrible crimes which atheists have committed in the 20th century. They are only assumed to be atheists, though, because people can't imagine Christians doing such things; in reality, Hitler explicitly appealed to Christianity on a regular basis and this was part of why he was popular. Not every Christian supported the Nazis, of course, but he was most popular with conservative Christians seeking a restoration of traditional values.
In The Holy Reich: Nazi Conceptions of Christianity, 1919-1945, Richard Steigmann-Gall writes:
In a speech celebrating Germany’s exit from the League of Nations, Hitler again maintained that the Third Reich was actively implementing a Christian agenda: “Along with the fight for a purer morality we have taken upon ourselves the struggle against the decomposition of our religion. We have therefore taken up the struggle against the Godless movement, and not just with a few theoretical declarations; we have stamped it out. And above all we have dragged the priests out of the lowlands of the political party struggle and have brought them back into the church.”
This declaration was quite consistent with Hitler’s speeches earlier in the year and also with the basic attitude he laid out — privately as well as publicly — in the “time of struggle.” Insisting that Nazism as a state would not distinguish between Protestant and Catholic, he recognized only a common supra-Christian faith. True to his promise, Hitler defended Christianity against the “Godless” movement, outlawing the Socialist and the Communist parties very early after the Seizure of Power.
Now, one can argue that Hitler and the Nazis only appealed to Christianity as part of a political ploy — that they emphasized Christianity in public without ever intending to promote Christianity in reality. Such an argument would be accompanied by the claim that the actions of Hitler and the Nazis didn’t reflect “true” Christianity and, therefore, must be attributed to atheism, paganism, or something else.
There are two problems with this. First, there is little to no evidence that Hitler and his top leaders only endorsed Christianity in public and for public consumption. Their private remarks on religion and Christianity were generally the same as their public remarks, but they didn’t hesitate to privately contradict public remarks on other matters, like peace with the Soviet Union. The similarity of their public and private positions on religion and Christianity indicates that they were genuine.
Second, the above argument could be made about any of the crimes committed by Christians over the course of history. It’s ultimately an example of the No True Scotsman fallacy: no true Christian could do such things or advocate such things, therefore they weren’t true Christians and their crimes cannot be attributed to Christianity. This is a fallacious argument because it relies on shifting the definition of "Christian" to match whatever conclusion the person prefers.
The actions of Hitler and the Nazis were about as “Christian” as the actions of people during the Crusades or the Inquisition. There were certainly non-Christian Nazis, and several leading Nazis preferred a neo-pagan theistic religion over Christianity, but the position was never officially endorsed either by the Nazi Party or by Adolf Hitler himself. Indeed, Christian complaints about the paganism of some Nazi leaders were given a sympathetic reception.
Christians may not like acknowledging that Nazi actions might have anything to do with Christianity, but Germany saw itself as a fundamentally Christian nation and millions of Christians in Germany enthusiastically endorsed Hitler and the Nazi Party in part because they saw both as embodiments of both German and Christian ideals. Conservative Christians who wanted a return to traditional values either voted for the Nazis or one of the other right-wing nationalist parties which eventually supported and merged with the Nazis.