Well, the most commonly referenced work (by commonly referenced I'm talking ad nauseum) is Hitler Table Talks. Which include such bombshells as:
* "The best thing is to let Christianity die a natural death.... When understanding of the universe has become widespread... Christian doctrine will be convicted of absurdity.... Christianity has reached the peak of absurdity.... And that's why someday its structure will collapse.... ...the only way to get rid of Christianity is to allow it to die little by little.... Christianity the liar.... We'll see to it that the Churches cannot spread abroad teachings in conflict with the interests of the State."
* "The reason why the ancient world was so pure, light and serene was that it knew nothing of the two great scourges: the pox and Christianity."
* "Christianity is an invention of sick brains: one could imagine nothing more senseless, nor any more indecent way of turning the idea of the Godhead into a mockery.... .... When all is said, we have no reason to wish that the Italians and Spaniards should free themselves from the drug of Christianity. Let's be the only people who are immunised against the disease."
Clearly, one should have little doubt of Hitler's atheism and hatred for Christianity! And the only issue that I can raise is that it's pretty much all lies and a hoax.
I strongly recommend Richard Carriers work on the subject from the 2002 November issue of Freethought Today: On the Trail of Bogus Quotes.
Sadly, due to the brilliance of Carrier I'm hardpressed to summarize his statements better than he, so here's a few choice quotes. Though I recommend reading the original article and, if so, inclined his piece in German History Studies.
"For example, one oft-repeated quote comes from 13 December 1941: "But Christianity is an invention of sick brains: one could imagine nothing more senseless, nor any more indecent way of turning the idea of the Godhead into a mockery" (Stevens and Cameron's English, again matching Genoud's French verbatim). But the original German says, "Christianity teaches 'transubstantiation,' which is the maddest thing ever concocted by a human mind in its delusions, a mockery of all that is godly." The difference in meaning here is radical, and again shows how Genoud (hence the Trevor-Roper translation) has distorted Hitler's criticism of one form of Christianity (which implies he believed there was a true Christianity) into a thoroughly anti-Christian sentiment."
The reason why the quotes appear to be a caricature of what right wing Christians might *want* Hitler to say... because that's pretty much what they are!
Stevens and Cameron are certainly guilty of some shameful incompetence, if not outright dishonesty. Nor does Trevor-Roper have much of an excuse. But the real culprit is François Genoud. David Irving tells how Genoud attempted to hoax him in the 1970s with a forgery of "Hitler's Last Testament."7 Genoud even confessed the forgery to Irving, declaring in his defense, "But it's just what Hitler would have said, isn't it?" He was evidently willing to perpetrate a hoax, thinking it permissible to fabricate the words of Hitler if it was what he believed Hitler "would have said." His motives for doctoring the Table Talk may be unfathomable. Genoud was a very strange man with a colorful history: a Swiss banker and Nazi spy who laundered money for the Third Reich, a self-professed neo-Nazi even up to his suicide in 1996 (though, stranger still, he never supported the holocaust), a voracious purchaser and profiteer of Nazi archives, and an admitted financer of terrorists
Sure, Hitler may have not been anti-Christian (although he did disagree with some dogmas and some church practices / who doesn't?) but 'it's just what Hitler would have said, isn't it?'
I am often astounding what a small amount of scratching can do to key pieces of evidence for commonly repeated Christian claims. This is probably more due to the fact that they become "key" when there aren't other sources saying the same thing and with general or common claims there should be many independent sources for the information at hand. However, it's odd to see how aptly lies are bought hook line and sinker. I suppose it's hard to avoid believing false things ever. Still, some people don't even try.