My personal opinion is that clear truth backed by evidence needs less defense than religious belief. Fervent Christians must indoctrinate their children and edit out anything offering contradictory theories in the hopes that the religion will "stick" long enough to become ingrained.
I have a roommate who was originally on the verge of an argument anytime I watched an television program about the evolution of humans (or evolution in general or anything else which discussed an ancient earth) but who has mellowed enough to confess that his years in Catholic school gave him a foundation of belief which would be shattered if he simply accepted what I believe as truth, and he has found comfort in his religious teachings that he does not wish to be without by abandoning them.
Perhaps the ease with which non-religious explanations of life and this planet were formed and continued can be taught, alongside copious evidence, similarly engenders vulnerability in those who were taught religion as the be-all and end-all explanation. Perhaps people don't want to lose the feeling that everything their parents told them was true, even if they start to realize it isn't. That Atheists can be happy and content is a threat to the insecurely religious.
Additionally, religions are "clubs." Christianity has been all about "spreading the word" since the beginning. Atheists are for the most part not organized, since by definition they are merely rejecting the notion of religion. It's easy to get a "club" together of people who believe the same set of rules, but that doesn't mean the non-members are automatically in another "club."
I know this is going to sound derogatory, but I'm being honest about my own beliefs. Personally, I think that the lack of a god is true, and religion is just a bunch of B.S. that got sold over time. I look at those who believe in god the way I look at children who believe in Santa Claus and the Easter Bunny -- "aw, how cute...you really
do think there is one." I don't think it's up to me to destroy their beliefs as long as they're kept personal and not forced upon me. If a god-believer were to challenge my beliefs, however, I take off the gloves. I will not get vicious, but I will not hold back on sharing what I know to be facts about science and about the histories of religions that I've learned, despite any destruction of false security he or she may have.
I was reading
God Is Not Great: How Religion Poisons Everything by Christopher Hitchins as my train book for a while, ironically around Easter last year. A woman sat on the train next to me and took it upon herself to try to educate me on the errors of my ways by not believing in god and Jesus. She wanted me to come to her church to be saved. I could go into detail about how the rest of that train ride went on (a completely civilized discussion between two strangers for almost half an hour), but this post is already too long. I wondered why she felt it was ok to challenge my beliefs because of my book, and if it would be just as ok for her to have the same discussion with someone reading the Koran or in obvious Orthodox Jewish attire. Somehow, Christians trying to save Atheists is not as offensive as Christians trying to save believers in other religions.