Why are some moderate religious afraid of atheism?

[Der/die AtheistIn]

Some moderate religious I know or heard about use their religion more for comfort than as a guide. They might use it as a guide, but to a lesser extent. I've noticed that many of them get very defensive if you tell them they should leave their faith. When I say defensive, I don't necesarilly mean violent or mean, some are both polite and defensive at the same time. Maybe they're not expressing it out loud, but they seem scared to me. Also, they get nervous if someone close to them wants to leave their faith.
I'm talking about religious that don't believe in eternal punishment. Using religion for comfort and not wanting to leave it is one thing, but being afraid of atheism is too much. They have the Right by law to have a religion, then why are they nervous if someone disagrees with them? Why are they nervous if someone close to them leaves the faith? Do they think that the said person will be unhappy? How do they know that the said person is unhappy? And if the person can't resist without religion, they can return to it whenever they want. 
I get why fundamentalists are afraid of atheism, but why are some moderates too?