/u/kieslowskifan has previously addressed the specific case How did David Irving continue denial when witnesses existed?

/u/commiespaceinvader (with /u/Elm11, among others) were part of a Monday Methods post about Holocaust Denial and how to combat it

People deny the Holocaust happened because they are antisemitic. Hatred of Jews is the core of it, no matter what else deniers say or use to dress it up. Holocaust denial is not about facts or seeking the truth.