"Weren't all of Hitler's followers Christians and wasn't Germany before and during Hitler's rule an almost exclusively Christian nation?"
On paper, sure. The same could be said of Lenin at the start of his revolution.
And as with Lenin, Hitler and the Nazis leaders intended to change that.
Events intervened, so that they were prevented from instituting changes as dramatically and swiftly as possible. But there is no serious doubt that Hitler saw Christianity as an enemy which he wanted to uproot.
People mistake Hitler's Mein Kampf as the sacred/guiding text of the Nazis revolution. It wasn't. (Though it is anti-Christian enough.)
Alfred Rosenberg's "Myth Of The Twentieth Century" is really the Ur Nazis text, which rests on the shoulders of Chamberlain's work:
The Myth of the Twentieth Century
http://en.wikipedia.org/wiki/The_Myth_of_the_Twentieth_Century
Rosenberg, like Hitler, saw Christianity as just a further flowering of Judaism.