The blatantly false claim that Hitler was a Christian is common in atheistic apologetics. In a vain attempt to pin the Holocaust on Christianity, thereby demonising the religion (because everyone knows a religion is defined by something that happened 1900 years after its inception), many atheists point to the slogan "Gott Mitt Uns" (God with us) on the belt buckles of the Wehrmacht (the German armed forces). However, this is as meaningless as trying to say all Americans are Christians because the phrase "in God we trust" is printed on their money.
In reality, the Wehrmacht had been using this slogan since before Hitler was born in 1889, since it was inherited from Kaiser’s Imperial standard of 1870. Wehrmacht soldiers were also legally forbidden from being members of a political party, so since the Nazis were a political party by definition, the slogan certainly cannot be attributed to the Nazis. Furthermore, the Nazis actually replaced the slogan with "Meine Ehre heißt Treue" (my honour is named Loyalty).
Furthermore, while there are a few scattered quotes of Hitler claiming to be Catholic (which isn't a Christian religion anyway), he never really acknowledged the Lordship of Jesus, whether in his own life or in general. This isn't even remotely surprising from the most notorious anti-semite in History, Jesus being a Jew and all, but we know from more of Hitler's statements that he hated Christianity. He wanted to eliminate what he called the "Judeo-Christian pestilence", and even used the Catholic doctrine of transubstantiation to express his disgust at the fact that it still existed.
But even lacking Hitler's quotes about his desire to get rid of Christianity as soon as the war was over, it's pretty obvious from his own slogan that Hitler wasn't interested in letting God have any say in Germany's government. The phrase "Ein volk, ein reich, ein fuhrer" can be found in many of Hitler's speeches. I even remember distinctly in history class, we had to write a Hitler speech, and it had to include that phrase. The absence of "ein Gott" in this phrase is enough to show how little Hitler wanted to include God, or at least to show a Theocracy was not his goal, but anyone with a more than cursory understanding of Christianity would be able to tell you that this phrase alone is actually contrary to Christian teaching.
Ein volk (one race)
Whereas the Bible teaches that race isn't so much of a concern to God, Hitler was not fond of multiculturalism. Having accepted Evolution, Hitler was a huge fan of Social Darwinism. In his eyes, Germans were the superior race, and should wipe out the others, just as Darwin had predicted the Caucasian race eventually would. No educated Christian could ever accept such an abominable view! Certainly not to the extent of trying to kill off the Jews, given that God had promised Israel would one day become a nation again (which, ironically, World War 2 set in motion). The Christian view is that race doesn't even really exist, at least not as Hitler understood it, given that all humans are descended from the same original pair. We may come in different colors, but we certainly didn't come from different apes.
Ein Reich (one empire)
When Jesus said "my kingdom is not of this world", He intended His followers to understand that we don't need to fight to establish a global empire. That doesn't mean we can't form, serve, or revere earthly empires, but at the end of the day, Christians can be content with tiny, insignificant communities while we're waiting to be called home to Heaven. But Hitler had his greedy sights on far more than Jesus died to buy him. He didn't want the Kingdom of God, he wanted the Kingdom of Hitler. A world governed entirely by Germany. Greater than the Roman Empire, the Austrian Empire, the Russian Empire, even the British Empire. Hitler literally wanted to take over the world, which would be an incredibly lofty goal for someone who believed Jesus had said such a goal was worthless if you lose your soul (Matthew 16:26, Mark 8:36, Luke 9:25).
Ein Fuhrer (one leader)
A time will come when Jesus will rule the Earth in peace for 1,000 years, but Hitler wasn't speaking about Him. Hitler wanted to establish himself as the first Fuhrer, followed by his successors, whom he would presumably have hand chosen. In Hitler's vision, there was no room for God.
It is clear, then, that Hitler was not a Christian. If he was, his campaign would have been significantly different (and, given the sin-stained pattern of the world, likely less successful). Rather than seeking a single race to lord it over the others, he would have sought the equality of all people. Rather than seeking to build an earthly empire, he would have sought to build the Kingdom of God by creating a very Gospel-friendly society. Instead of seeking to be a dictator, he would at the very least seek to be little more than the deciding factor after hearing a variety of wise, Godly counsel. It is honestly astonishing to me that people try to pin the Holocaust on Christianity despite the fact that not only did Hitler never claim to be a Christian, but also went strongly against the faith by word and by deed.