I don't personally care what people believe in the privacy of their own homes and hearts. What I care about is ensuring that we all have freedom of conscience. No one religion should ever have dictatorial power over public policy.
Some say "Atheism is a religion too," but that's only true if you define "religion" as "way of life". Otherwise, it has none of the hallmarks of organized religion. Being someone raised in an Atheist home, I'd define "Atheism" as exactly what it is -- absence of deity, hard stop.
The major difference between a governmental policy based on religion vs one more secularly-based is that Atheism concerns itself with the concrete here and now, based on verifiable scientific evidence. To me, that's a far stronger foundation for law than any unverified personal belief. Just like in a court of law, hard evidence is the standard, rather than some person in a collar saying "God told me so," so too must our laws be held to a standard that treats everyone on a level playing field.
Ultimately, a secular basis for law is the most equitable, because it is based in concrete evidence. As I've stated before, the moment we start giving preferential treatment under the law to any one organized religious sect -- a sect based in magical, rather than evidential thinking -- all other religions' freedom of conscience is endangered. This was the whole reason behind the Constitutionally-enshrined Separation of Church & State.
Now, a common counter-argument is the mistaken belief that Atheists, lacking a Deity figure to dole out punishment for bad behavior, have no morals. This argument, while I'm sure is appealing to the more Regressive Religious crowd, is also entirely false.
A good example of this line of questioning -- "What scientific reason can be given for say helping out your fellow human being that's stuck under a car especially if it puts your life in danger?" -- is very easily answered with the simple litmus test of reversing the circumstances. If I were stuck under a car, I'd want someone to help me out. Therefore, I help out when I see someone else stuck under a car. No need for deities in that equation, just simple empathy and the ability to put myself in someone else's shoes.
What I find interesting in many of the conversations I get into is how threatened Theists are by the very existence of Atheists. Why is a person saying "there is no God" such a threat, unless on some level of the Theistic mindset there's an agreement with that statement -- or the fear that, because such beliefs could be mere products of thought and thus erasable by the appearance of a contradictory thought? Can truly deep-seated beliefs be so easily uprooted by someone else saying, "it ain't necessarily so"? What is it about one person's refusal to believe in a particular tradition that so threatens those who do? There's a very real insecurity going on here.
I suspect it's from a knowledge of history -- Theistic religions have for generations literally burned Atheists at the stake for disagreeing with the State Religion. I think there's an inherent fear that Atheists will return the favor. If empathy holds as a human trait, I highly doubt that will be the case.