Hi, there have been several lengthy threads on Holocaust denialism. Take a look through these, and if you have follow-up questions for any of the users, just ask here and tag their username to notify them.
When and how did Holocaust denialism start? by /u/commiespaceinvader
Monday Methods: Holocaust Denial and how to combat it by /u/commiespaceinvader
Holocaust AMA. See, in particular, this comment by [deleted]
Historiography: how responsible has postmodernist theory been in creating the intellectual conditions in which modern Holocaust denial thrives? by /u/restricteddata, /u/BonSequitur, et. al.
Does sensitivity to being labeled a "Holocaust Denier" limit our ability to research and discuss the Holocaust? Are there other topics that have similar sensitivities? by /u/sowser, /u/gingerkid1234, et. al.
ELI5 - Holocaust Denial by /u/Elm11