It would have to be a massive conspiracy seeing how the proof of evolution is pretty widespread.
Also Satan is one evil entity in one religion. If someone said that Loki and Apep were conspiring to do this would you believe those things? These good and bad dieties are cultural.
Also human quality of life has gotten a lot better since the industrial revolution. The black plague and smallpox are pretty much gone. Most people will take health and wealth over the approval an arbitrary religion.
This is a non-testable hypothesis. Satan convinces people he doesn’t exist and uses mass hypnosis, you can create any philosophy if you are going to say ‘group X convinces people they don’t exist, then use hypnosis to back up that claim’.