Why do you believe in a God?

[Lek]

an idiosyncratic belief or impression that is firmly maintained despite being contradicted by what is generally accepted as reality or rational argument, typically a symptom of mental disorder.
"the delusion of being watched"
synonyms: misapprehension, misconception, misunderstanding, mistake, error, misinterpretation, misconstruction, misbelief; More
the action of deluding someone or the state of being deluded.

How would you know?

I would ask you the same question. Are you sure you're not just imagining all this? Just calling theists deluded doesn't get you anywhere. I'd be more considerate of the testimonies of rational people instead of just calling people derogatory names.