Harrison's Law is used in relation to beliefs or faith but does not have to involve god or religion.
"Any belief that is based on no evidence is equal in rational validity to any belief you can invent on the spot."
Harrison's Law us used to demonstrate to an irrational believer that their belief is completely unfounded in a way which makes sense. If you consider a belief that has no evidence to support it such as a belief in Zeus, then this belief is as rational and logical as believing in anything you can invent yourself.
The law is used to show "believers" that their belief could be anything at all and would still carry as much meaning, validity or usefulness as any fictional belief made up there and then.