Why so certain?

I've heard from a number of Christians that there is no possibility of them changing their views. That I am wasting my time in even trying to discuss it with them. This seems to me to be a fairly counter productive position to take, and seems almost designed to stop further conversation. Even accepting that, however, I still find that I want to know why this person is so certain. What is so convincing that the person is willing to state, unequivocally, that nothing could change their mind. So, what is the answer? That's what this post is for - you tell me. Educate me as to why you have such certainty. Theists and atheists alike (and agnostics, and anyone in between) - if you are certain of your position, and/or believe you could not be argued out of it, explain why? Or, tell me of reasons you have heard from others, regardless of their beliefs.