A Jew can fool even himself for a very long time, but in the end, he can never avoid being a Jew. One of the tasks of gentiles is to make sure of that, though free will determines whether such prodding awareness is good or evil.
Not quite sure what you mean by a Jew, are you including those with Jewish parents who are Christian. Like Marx, or John Allen or Barry Goldwater, none of whom I’d consider Jews, though the first two would qualify as heretics. Goldwater, like Kerry, would only be Jewish enough to be killed by the Reich. In my mind, conversion to another faith is the end of it, though I recognize it impacts returning to the faith. Not sure I understand the role of gentiles, they’re not really relevant to the question of Judaism.