It’s called “projection”. We never really know what it’s like to be another person. We interpret another’s behavior in the light of our own motives, values, emotional responses and other mental processes we experience directly. We know ourselves (somewhat, The more we know ourselves, the better we can begin to understand others. ), so we can ‘project’ our own feelings onto others as a way of understanding them. I think it’s fair to say that atheists and theists have different mindsets. Therefore, they have a hard time understanding each other. When a theist says that atheists are ‘pompous prigs’ and ‘arrogant’, he is attempting to understand his opposite using his own thought processes to model the other. Seen in this light, theists who interpret atheists in these ways are really revealing something about their own thought processes.