The atheist was David Silverman, president of American Atheists, who was on to discuss a billboard put up by the organization that reads "You KNOW they're all SCAMS" – "they" being religions. First there was a discussion over whether the word "scam" constituted an insult. O'Reilly thought it did; Silverman considered it merely a true statement. I'd consider them both right: it's a true statement, stated in a way that insults religious people.
The statement that most amazed me with its stupidity came from O'Reilly. In defending his belief in God, he said: "Tide goes in, tide goes out, never a miscommunication. You can't explain that." Um... actually, we have a pretty good idea of how tidal forces work. It's simple physics. O'Reilly's use of the phrase "never a miscommunication" reveals his assumption that phenomena like these absolutely require a supernatural being pulling the strings to make them work. Silverman sarcastically suggested that "maybe it's Thor on top of Mt. Olympus" who's doing it, which helped underscore the absurdity of the argument (bonus points for mixing mythologies). O'Reilly also asked "why" the tide goes in and out. For religious people it seems there always has to be a "why," a meaning and intention behind everything that happens. In reality, there is no answer: the question itself is flawed.
Silverman did say a few silly things, though. He tried to claim that "everybody knows religion's a scam" – a ridiculous assertion that went more or less unsupported. He also claimed that "the churches are filled with atheists," which is clearly a massive exaggeration; most churchgoers are there because they believe what's being taught. When asked for reasoning, he initially said "we don't know," which O'Reilly (and my dad) seized upon triumphantly. But neither of them seemed to notice his subsequent mention of "familial and social pressure," which was spot on. In fact, familial pressure is precisely the reason I go. O'Reilly called him a "loon," but although Silverman's claim was overstated, there's undoubtedly a grain of truth to it as well.
At the end of the discussion, O'Reilly asked what good it did for the atheist minority to "attack" the theist majority. Silverman replied that the real intention was to "lure the atheists out of the pews." It's not clear whether that's an effective strategy, but it does make some sense. Unbelievers often feel isolated in their unbelief, and a billboard criticizing religion shows that there others like them, which may in turn embolden them to be more vocal about their unbelief.
Overall, I wasn't that impressed with Silverman's performance. He made some good points, but his exaggerations were annoying and probably alienated a lot of viewers.