Explaining to atheists why Adolf Hitler cannot have been a Christian feels as strange to me as to have to explain that the sun is not made of ice. It should be common sense that history's most notorious anti-semite did not worship a Jew as his God. Nevertheless, many atheists do claim Hitler was a Christian, and respond to all attempts to explain otherwise by accusing their opponents of making the "no true Scotsman" fallacy.
The no true Scotsman fallacy, otherwise known as appeal to purity, is not merely the denial that X is Y. Rather, it is the selective definition of Y in order to exclude examples of Y that are inconvenient to one's argument.
The classic example, which gives the fallacy its common name, is of a Scotsman named Hamish. Hamish reads about a serial killer in the newspaper, and declares "no Scotsman would do something like that". He reads on, and discovers that the killer was, in fact, from Scotland. Hamish then adjusts his statement, saying "no true Scotsman would do something like that." This is the no true Scotsman fallacy, not because Hamish merely denied that the killer was a Scotsman, but because Hamish has self-servingly defined "Scotsman" in such a way as to exclude all serial killers who are Scotsmen. In reality, there is no meaningful definition of the word "Scotsman" that would necessarily exclude serial killers.
Hitler as our example, there is no meaningful sense in which Hitler could be described as a Scotsman. You can literally say the words "no true Scotsman..." and describe any relevant attribute of Hitler (e.g. he never so much as visited Scotland), it would not be the No True Scotsman fallacy.
The question, therefore, is whether or not there is any meaningful definition of "Christianity" that would apply to Hitler? The answer, of course, is no. Christianity, as the name suggests, is all about Jesus Christ, a Jew. If Christianity can be defined so ambiguously as to consider a man a Christian even if he would have sent Jesus to die in a gas chamber, the word ceases to mean anything, because it begins to mean literally everything.
In reality, Hitler cannot have been a Christian, because whereas Hitler is most notorious for his hatred of the Jews, Christianity is a notoriously Jewish faith. We have the Jewish history, we believe the Jewish scriptures, we worship the Jewish God, we were saved by the Jewish Messiah, it is almost fair to say that Christianity is Judaism. Hitler, by his assault on Judaism, was about as anti-Christian as one can possibly get. And he did not intend to stop there. His post-war plans for Christianity were pretty much identical to his actions during the Holocaust.
But the amazing thing about the Jewish God is that He loved even Hitler. Whereas Hitler hated Jews to the extent where he would have attempted to take Jesus' life, Jesus actually laid down His life willingly, on the cross. Through His death on the cross, Jesus paid for the sins of the entire world, purchasing the forgiveness for them. All one has to do to be forgiven is to repent of those sins and confess Jesus as Lord, believing that He rose from the dead. Hitler never accepted this gracious offer, and so he is now doomed to suffer the full penalty for his sins. You do not have to do the same. While there is still breath in your lungs, blood pumping through your veins, and you retain the ability to make conscious decisions, you can repent of your sins, confess the Jewish Messiah as Lord, and believe that God raised Him from the dead. You do that, you will live forever.