Characteristics of Religious Experience
According to William James, Religious experiences are...
Ineffable - The experience is not able to be described in words. If this is to be true then it would surely mean that the only way to understand a RE is to have one, as other peoples' accounts would not describe it adequately.
They are also a very personal event, and vary according to the person experiencing them. This idea means that other people would not be able to understand someone's religious experience as it was personal to them and only them.
Effects of Religious Experience
Althoug't if someone has not had a RE themselves, it doesn't mean that they cannot understand it in terms of the after-effects it has someone else's life.
It may not seem possible to understand what it feels like to have a RE, but it can be seen from observation the profoundly life-enhancing effect that RE has on other people.