As always, every denomination has its own theology regarding how one identifies those who are Christian. Very few of them equate ‘church membership’ as the important issue, and most will not affirm that church membership equals salvation.
Jesus himself affirms in parable that there will be weeds sown amidst the wheat, that there will be a time of separating sheep from goats, and that many call Him Lord but He’ll tell them to depart from Him for they are nothing more than workers of iniquity.
I do not determine any man’s salvation, but the bible does tell me that by their fruits I will know them.
Hitler’s fruits argue that he was not a Christian.
His fruits yes, but he also gave up Christianity to replace it with an “Aryan Jesus”