Why do some religious believers seem to take it personally when you or I say, "I am an atheist," almost as if we have just insulted them by making such a simple statement about ourselves? I don't have any profound answers to this question, but it seems to me that "I am an atheist" often means something very different to many religious believers than it does to us. In fact, I have come to suspect that it may mean something like the following to many:
You are wrong about gods, and you have wasted a significant portion of your life devoted to groveling before figments of your imagination. There will be no afterlife, no reuniting with loved ones, and no salvation for the "sins" you think you have committed. Every moment you have spent in prayer has been squandered because there isn't anything out there altering the universe for you. You have based much of your life on a lie, and you are a moron for doing so. I reject your values, your morality, and everything you hold dear.
If that is anything like what some religious believers hear when you or I identify ourselves as atheists, it is understandable that they might take it personally, get defensive, or have other negative reactions. Now, I must admit that while I have never met an atheist who would say that "I am an atheist" means any of this to them, I have met a few atheists who would do admit to holding these attitudes about religious believers. Some of them even express these attitudes to religious believers with the same lack of sensitivity which I used in the statement above.
While I cannot deny that there are atheists out there who feel this way, I believe that most atheists have less negative attitudes toward religious believers and express their views with a bit more tact. And so, I suspect that when religious believers take a simple statement about atheism personally, they are doing so because it means something very different to them than it does to us.