I_olive, I was a fundamentalist Christian for over half my life so I understand the mindset.
It's not that they feel threatened (or at least I didn't) but rather a conviction that there is only One Truth which naturally enough happens to be their particular belief. If there is only one truth, and they have it, then obviously anything different is wrong. Add to that a belief in Satan who is trying to deceive people so they'll go to Hell. So evolution is a plot by Satan to lead people away from God. All other religions and religous experiences are nothing more than counterfeits created by Satan.
I don't quite know how to explain this, but to maintain this pretty simplistic worldview in such a complicated world, one has to impose it on EVERYTHING. If something comes up that doesn't fit with the worldview, one has to disbelieve it. So for example when a fundamentalist meets someone like me who is no longer a Christian and very happy with my situation (when I should be miserable and convicted), they'll think to themselves (or sometimes tell me outright) that really truly deep inside I know the truth, and I am just resisting it because I don't want to abide by God's laws/am selfish/being deceived by Satan etc. I could tell them until I am blue in the face that this is not the case and they won't hear it.
Oh, I suppose for those that believe the Bible is inerrant and literally true there could be an element of feeling threatened. Because if any jot or tittle turns out to not be true, it will shake their entire faith. I remember as a high-schooler frantically trying to make the various geneologies in the Bible match up with each other, because if those were wrong then I would lose my faith.
I have to say, after participating in this thread I am so glad I am no longer a part of that. It's great for some people, but not for me.
This thread makes me glad, too -- glad that I've never felt it necessary to denegrate anyone else's beliefs in order to preserve my own. Because it would, indeed, be hard for me to reconcile my moral and religious convictions with a kind of Christianity which required its members to be arrogant and narrow-minded.