Theists, are you immune to being decieved?

[Kitan Etos]

So of what behavior is delusional while other behavior is not?

Delving into psychology would most likely answer this question, except that I would take it one step further by stating that the individual's behavior can still be harmless to one's self as well as others for the delusion to still exist.

Maintaining that something exists despite the obvious knowledge that it does not is a clear sign of delusion.

Or are all assumptions presumed to be equally grounded in reality?

Assumptions are just that, an opinion derived not from fact but from guessing. Just as psychics sometimes get some things right by chance, assumptions sometimes have the chance of being correct as well, but to state that assuming is a science would be ignorant.

Having faith that a deity exists is merely assuming that the deity exists despite the foolish theistic claim to knowledge.