It's not aggressive to question why someone believes in one thing or another. It is perceived as aggressive by the religious set because they are simply unused to people having the temerity to question their motives and/or logic. They expect people to automatically 'respect' their faith, not question it, regardless of how ridiculous or unlikely that belief may be.
I remember reading a couple of months ago in the back of a TV listings magazine, a letter from a woman who objected to a presenter on a show about guardian angels (not the vigilantes from America – actual guardian angels). The woman's objection was that one presenter was sceptical about the existence of angels whilst all the other presenters accepted their existence as a fact. She said something along the lines of, “I believe in angels and this presenter's comments are ruining the show for me. Why can't the sceptical presenter just keep quiet?”
And that, in a nutshell, is the attitude of many people who believe in God or some other form of spiritualism: if you don't believe, don't criticise or ask questions – just keep your mouth shut and go away.