People who circle the wagons when questioned about their religious creed are usually afraid that what they profess might not be true.
Seldom (if ever?) will you run across 100% false belief system. There are scattered nuggets of truth in each one. That’s why there are people in every religious, political, and philosophical system who simply accept the group’s views at face value. They grew up in it, they gave in to social pressure and they joined. Or else they simply are unwilling to come to terms with the fact they have been walking on the wrong road. Admitting that you’ve invested yourself in something that’s been a waste of your time is not easy. Going back and starting over again is not very appealing. Ultimately, it’s a choice about whether to maintain a comfort level or pursue truth.
If you surround yourself only with things and people who reinforce your belief system, you don't have to worry about your worldview being knocked out from under you (although circumstances have a way of eventually doing this, anyway). The choice ultimately becomes denying reality--or reassessing cherished ideas on which we’ve built our lives.