This is a common misconception.
Hitler was born and raised a Catholic but rejected his faith very early on. There are a number of quotes that are circulated in books and the internet that claim to prove he was an ardent Catholic/Christian but they are usually taken out of context.
Hitler actually believed that all organised religions were wrong but recognised that they could be used as an effective tool to indoctrinate the masses. He used a number of religious arguments to justify his policies (e.g. killing Jews was acceptable because they crucified Jesus) but ultimately he had no genuine belief in religion.
As he increased his power base and grip on the people, he became a bit more careless and revealed his true beliefs. He even planned to eradicate Christianity and replace it with his own 'state religion' where the position of 'God' was replaced by the State. He planned to use his own book ('Mein Kampf') as the 'bible' of his new religion and the symbol of the crucifix was to be replaced by a sword, symbolising the battle against enemies of the State.
I can assure you, Hitler was definitely an atheist.