I don't remember the term for it, but psychology states that people have an extreeme confidence in what they would do if in a situation, that is rarely in scale with what they do if they ever actually end up in the situation. (People think they are better, smarter, faster, kinder etc than they are).
I truely hope I would be willing to die for my faith, and I think I would, but my life has never been threatened, so I don't know what it's like.