Adolf Hitler claimed to be a Christian, but would it be wrong to say he most certainly was not a Christian?
So far as we know, Adolf Hitler was validly baptized in the Catholic Church. That means he was a Catholic. Baptism is, literally, a new birth that makes the person a Christian in his very being, no matter how well or how poorly he lives out his faith. Just as physical conception means that a person will always be a human person with inherent human dignity, no matter how detestable the crimes he may choose to commit, so a baptized person, no matter how evil he becomes, remains a Christian. In Hitler’s case though, and in the cases of those Christians who also entirely abandon the faith into which they were baptized, it can be said that they no longer believe in Christianity and that their theological beliefs cannot be considered Christian. If they completely abandon their Christian faith, then they are apostates (cf. CCC 2089), though objectively they remain among the baptized.