You are correct about Hitler...and others as I noted earlier this morning.   Does a Jew who converts to Christianity cease to be a Jew? I would think yes, at least religiously, if it is a sincere conversion. But, again, you can find lists of Jews in various fields of endeavor who had themselves converted, or whose recent ancestors had converted from Judaism; Disraeli and Felix Mendelsohn come quickly to mind.

Please note that I do not speak for Judaism. I only speak from history, and the teachings and discussions I have understood over the years. There are many introductions to Judaism which can give a great overview, but if you read two of them, you'll get three opinions. :>\