As far as QAnon goes, this would be like discovering an unsecured laptop that actually belonged to the original Q and had something like IRC channel logs that they used when generating "what won't they believe" type ideas.

As for Scientology and Mormonism, they don't have explicitly historical and rational claims to truth like the Catholic Church and some sects of Judaism do. Ask a fervent Catholic or Orthodox Jew why they believe what they do and they'll say "Because it's true." As far as I know (I don't spend significant time with Mormons or Scientologists), they don't make this type of claim to be historically validated by peak reason.