who is a Jew?
Whoever was deemed as Jewish by the Nazis.
Like with global warming.
This is actually a good example on how that does not work. Oil companies tried their best to bury the evidence, financing studies that tried to seed doubts but now global warming is accepted as a fact by the scientific community.
Holocaust denial is an exercise in futility. Because there is simply no denying the many written sources that describe it. Which is why no Holocaust denier has ever attempted this, since they know there is no good explanation as to how this could have been faked. We are talking about millions of documents, spread in various archives across all of Europe.
Letters from German soldiers to their families in which they allude to having seen mass killings in the woods. Private diaries in which they go into more details. Official division diaries in which the commander notes how they rounded up families which were suspected of being Jewish and executed them in the woods. Official letters to local authorities, telling them that the "evacuated" Jews to the Eastern Territories had perished. And so on.