It is not uncommon for atheists to claim that Hitler was a Christian simply because he claimed to be. There are three main problems with this.
First, Hitler actually claimed to be Catholic, as he was raised to be. While commonly viewed as the same religion, Christianity and Catholicism have several fundamental differences that make it impossible to class them as one and the same.
Second, like a typical politician, Hitler had both public and private views. He was ruling over a largely Christian population, so if he had begun his campaign by publicly announcing "Our epoch will certainly see the end of the disease of Christianity (...)"(1), he would probably have faced a bit of opposition. But by gradually eroding the Christian culture of Germany, he has a chance of succeeding.
But you don't need to know about what Hitler said publicly or privately to know that he wasn't a real Christian. Hitler could have worn a cross, carried a Bible and shouted "ich bin Christian!" every morning, his actions testified against him.
Paul, in his letter to Titus, spoke of men who would claim God's name, but their actions suggest they are lying (Titus 1:16). And indeed, in James, we read that faith without works is dead. In John 14, Jesus says if we love Him, we'll obey Him and keep His words, whereas if we don't, we won't. Matthew 7, Jesus says not everyone who says to Him "Lord, Lord" will enter Heaven, but only he who does the will of the Father.
The list goes on. Many Bible passages tell us that merely claiming to be a Christian does not make you a Christian. You can tell the difference between a real Christian and a false Christian by their fruits. Now, that doesn't mean that anyone who sins is inherently not Christian. 1 John 1:8 says "If we say that we have no sin, we deceive ourselves, and the truth is not in us." However, true Christians must struggle against sin, not be defined by it.
My favourite analogy here is the difference between a pig and a cat. A pig will happily roll around in filth, whereas a cat will do its best to escape and clean itself off. Hitler, and I mean this in more ways than one, was a pig. He showed no signs of repentance for his sins, nor did he attempt to serve God. He "rolled around in his filth" like a happy little pig. His actions are enough for anyone with even the most basic knowledge of Christianity to say that Hitler was no more of a Christian than those who released Barabbas instead of Jesus.
References
Bormann, Martin - Hitler's Table Talk, 1941-44