People always seem to need two things: something to fear and something to hate. It gives our lives meaning in a complacent society, with cushy jobs and all of our basic needs met without any struggle. Because we have no real, regular threats to our lives (like disease, starvation or predators), we need to make them up. It is a NEED that we have.
Some people attach onto religion as their own form of "sanity". People who do not have this attachment are threatening to them. They have no faith in "just people", they naturally assume that anyone who behaves decently in society is in fact religious, and they are positively terrified when they discover that some nice people are actually atheists? Without the rules of religion to guide us, what is preventing us from just going nuts on them with all of the stealing, beatings, sodomy and murder?