David Silverman, president of the American Atheists organization, is a prolific speaker and debater on the question of God and religion, as well as the appropriateness of religious sentiments in the political sphere.
I'm not interested in attacking the integrity of Silverman (for all I know it's an honest mistake), but he very often makes the spurious claim that roughly 50 million Americans are atheists. Well, the stats prove just the opposite. He gets this figure by recent statistics that state between 15 to 25% of Americans are unaffiliated with any particular religion. Take, for example, this Pew Forum article. Even more striking is a recent Pew poll in which 21% of self-proclaimed "atheists" believe in God! An additional 55% of agnostics believe in God. 92% of the total American population believes in God.
Now let's do some basic math. Let's round up and say the total American population is 315 million. 92% of 315 million is 289,800,000. That leaves the remaining unbelieving 8% at 25,200,000 - half of Silverman's claim. What he would need to rely on in order to back up his claim is a (currently unsupported) psychological factor in which an additional 25 million people just won't admit they're atheists, even on an anonymous survey.
Moreover, that's Silverman's best case scenario. It's not that a full 8% are atheists, but simply lack a belief in God. The new atheists like to define atheism as a mere lack of belief in God, but as even Antony Flew conceded in the 1970's (when he was still an atheist), who popularized this definition, such a definition is entirely novel. And of course, agnostics don't necessarily identify themselves as atheists. Atheism, according to the Oxford Dictionary of Philosophy is: "The theory or belief that God does not exist."