What were his religious beliefs? It's an important question, but less important than it has been portrayed sometimes.
Historically, answers to this question have been contaminated by the No True Scotsman fallacy so common in discussions about religion. Christianity is good, and Hitler was not good, so Hitler wasn't a Christian. Even though Hitler said he was a Christian, this is not true because he was a Nazi, and therefore not good, and therefore not a Christian (there's good reason to think Hitler's profession of Christianity was insincere, but we'll get to that later). You can see the same pattern of thinking among Marxist-Leninists: Bolshevism is good, Stalin was not good, therefore Stalin was not a Bolshevik. And Islam is good, and ISIS is not good, therefore ISIS is not Islamic. And so on.
Buddhism is good, and the Buddhist mobs attacking Muslims in Sri Lanka are not good, therefore they are not Buddhists. Hinduism is good, and the Gujarat pogroms by Hindus were not good, therefore the perpetrators were not Hindus. And so forth.
I will confine my answer to Hitler's actions and self-identification, rather than saying whether he was a 'true Christian', because per Wittgenstein, universal terms like Christian can only have family resemblances and not a single defining characteristic. Of course (some) Christians believe there is a single internally coherent and true version of Christianity (theirs), and all other interpretations are heresy. However I am not a Christian, so thankfully I don't have to pretend that there is one true interpretation, so I won't.
The other leg to understanding his beliefs is Hitler's actions. How did he behave towards Christians and other religious groups, and what were his future plans?
Now we've got that out the way, as I said we'll take a direct look at Hitler. First there's his public statements. They are pretty unambiguous: Christianity is good, atheism is bad. He said some rude things about Catholicism, but anti-atheism was the main thrust of his arguments. This wiki page cites quite a few of them; it was part of his attempt to discredit his Social Democrat and Communist rivals. All this worked, and he got the support of German Christians: the vast majority of those in his party were practising Christians. We know this for the simple reason that 95% of Germans were Christians.2 Even if every single non-Christian had been a Nazi member (many were Sozis or Commies, wherefore Hitler's anti-atheist propaganda) then mathematically they would still be swamped by Nazi-supporting Christians going by 1939 population figures. Note that per the above paragraph I am not making some essentialist claim that theoretical 'true Christians' supported Hitler, I'm saying that practical Christians, who lived, breathed, and physically existed supported him.
Many atheists were executed; not necessarily communists, just anyone who had vocally supported some party other than the Nazis. A small minority of Catholics and an even smaller minority of Protestants interpreted their religion as incompatible with the Nazis' rule, and were also executed. These people are very famous (and boasted about) so I won't go into detail, except to say there is nothing contradictory in persecuting Christians and atheists. Remember, the first Christians were executed by the Romans for being atheists, in that they denied the existence of the Roman gods. And as we'll see below, Hitler had his own gods.
Hitler's private statements were less friendly to Christianity. He viewed it as a religion of weakness, and planned to extinguish it after the war was over. However, it is not accurate to characterise him as a secret atheist. He believed in some kind of divine presence, which was guiding the German race to master the world.
That's how he saw himself, anyway. In practical terms he never had a solid grip on reality, and after a couple of successful annexations he went totally off the rails; he believed that he was God. Both his statements and actions back that up. Unlike many of his Nazi colleagues, he really had no interest in neo-paganism and did nothing to encourage it.
I said above that Hitler's religious beliefs are not as important as people think they were. Hitler is not the same thing as Nazi Germany. If Hitler was not a Christian, this does not mean that Christianity came out of the war with clean hands. People make a big fuss out of Bonhoffer and Niemoller these days, but the uncomfortable fact is that most Germans, and therefore most Lutherans and a good chunk of Catholics, supported Hitler partly because he portrayed his war as a war against Bolshevist atheism, and inflamed their long-standing antisemitism.
To the extent that these people were complicit in the crimes of the Nazi state, they did not see a contradiction between their crimes and their Christian identity and beliefs. This may mean they were 'not true Christians', but what use is the concept of 'true Christians' if most people who think they are Christians are just confused? Equally, what use is the idea of 'true Communism' if none of the Communist regimes in history have been 'true Communists'?