I can't say what the reason is for sure, but I've always assumed it is one of two things (or both combined). The first is that, to them, the evidence is so overwhelming that it is shocking someone could fail to see it. In this instance, they just aren't taking time to consider your point of view, or they don't want to. The second reason is because they might interpret your lack of faith (or religious association) as a direct insult to their religion. So, for example, I'm an atheist, and I imagine that telling someone that sometimes might come out differently. They might immediately think; atheists don't believe in god, therefore he is telling me that I'm stupid for believing in god. So, saying I'm an atheist is heard as "you're stupid for believing in god". I never said that, nor would I, but I think that happens a lot. They feel threatened by you and must prove you wrong, or crush you, in order to validate their own system of beliefs. Not all religious people act this way (and some non-religious people DO act this way) but that's just my assumptions based on what I've seen.