Hitler’s religious beliefs were obscure. At times he seemed attached to Norse Mythology and occult practices like astrology. He is reported to have said “Richard Wagner is my religion”. His speeches were peppered with religious terminology like “providence”, “salvation” and “the Lord’s work”, but, by his own admission, he was a thoroughly cynical propagandist. It can’t be stated with certainty that he was an atheist, but it can be stated that he was neither a Catholic nor any kind of Christian.
Hitler hijacked churches by stacking them with his own priests and pastors, and invoked the name of God in his speeches and writings, although he was clearly not a believer.
This even led to a disgracefully anti-Semitic and anti-Christian butchering of the Bible. This was so that he could fool many Christians into believing that he was actually on their side. But at the same time he secretly killed real Christians or threw them into the concentration camps, including the Christian White Rose resistance movement that came out of University campuses in Germany. Led by Sophie Scholl, who is a modern-day hero in Germany, these Christians actively campaigned against the Nazis and became martyrs in the process when they were guillotined for their beliefs.
It doesn’t really matter what Hitler said about “doing God’s work” in his public speeches, because politicians lie all the time in order to get votes from particular groups, or to maintain their popularity.
Perhaps a devious politician seeking power in a nominally Christian country, like 1930's Germany, would affect the trappings of religion to maintain popularity?
During his ascent to power, Hitler needed the support of the German people—both the Bavarian Catholics and the Prussian Lutherans—and to secure this he occasionally used rhetoric such as “I am doing the Lord’s work.”
To claim that this rhetoric makes Hitler a Christian is to confuse political opportunism with personal conviction. Hitler himself says in Mein Kampf that his public statements should be understood as propaganda that bears no relation to the truth but is designed to sway the masses.
And what did Hitler do once he had absolute power?
He denounced Christianity, proving that he was an opponent of Christianity!
The Nazis stopped Germans from celebrating Christmas, and the Hitler Youth recited a prayer thanking the Fuhrer rather than God for their blessings.
Clergy regarded as “troublemakers” were ordered not to preach, hundreds of them were imprisoned, and many were simply murdered. Churches were under constant Gestapo surveillance.
The Nazis closed religious schools, forced Christian organizations to disband, dismissed civil servants who were practicing Christians, confiscated church property, and censored religious newspapers.
Hitler's life and actions show that he was rabidly anti-Christian. You could not commit the crimes Hitler did and be a follower of Christ. If he was following a god and doing what he believed was "god's work", it was a false god and certainly not the God of the Bible.