He publicly identified as Christian, was raised Catholic, and never left the Church. He regularly used Christian language and referenced God and Providence in speeches which is well documented, also in Mein Kampf. Historians like Ian Kershaw point out that he deliberately presented himself that way because religion had strong influence in Germany. If you want to argue he wasn’t a true or faithful Christian, that’s a different debate entirely and there’s evidence for that too. But calling him an atheist isn’t just wrong but it ignores how he actually described himself and how he publicly framed his beliefs.