All fear is not an illusion.
Unreasonable fears planted by cults in the minds of thier victims is an illusion fostered and nurtured by destructive cults.
For example, the fear of leaving. The fear that leaving will mean death, certain damnation or have some sort of cosmic consequence.
Cults make people feel there is no legitimate reason to leave and implant fears about leaving to retain members.
Reading about how this works and educationg yourself about cults is the key to unlocking these unreasonable fears.