But for the purpose of this debate, the three philosophers decide, "God" will mean an Abrahamic God, a personal, anthropomorphic God, a "who" rather than a "what," a God who cares about humans, not just a creative force that may have set the universe in motion. Because Chatterjee, Keller and Hausam are philosophers, the debate will focus on arguments based on "reason" rather than those based on "faith." But they'll try not to throw around words like "epistemological."

Each debater will get 15 minutes to sum up his position, distilling centuries of arguments — from Thomas Aquinas' "Summa Theologica" to Richard Dawkins' "The God Delusion" — in what they hope is the most convincing proof, yea or nay.

Keller says he doesn't feel comfortable arguing categorically that he can prove God doesn't exist. How about if he just says "the vast preponderance of evidence rules against the existence of God"? It sounds so arrogant to proclaim that he knows for sure one way or the other, he says. "I'd have to have an almost God-like omniscience."

Chatterjee is clearly disappointed. "That leaves people clamoring for more," he says.

The three men talk about the difference between "reasonable" and "rational." "Lots of things that are irrational we hold onto, because they're reasonable," Chatterjee says, offering the example of Santa Claus. To show that believing in God is reasonable, he tells Mark, isn't good enough. "Your task is to show, based on evidence and reason, that it's rational to say that God exists."

Chatterjee grew up without any religion. "I'm still searching," he says. Hausam is an orthodox Presbyterian. Keller is descended from Mormon pioneers who came across the Plains with Brigham Young. He first began to question his religion as a teenager, when he noticed "that there was a striking dissimilarity between the way my peers and the people I observed behaved during the week and on Sunday." That disconnect between religion and ethics led in subsequent decades to the study of "all the arguments for the existence of God by all the great Christian philosophers, which I found to be incredibly fallacious."

"I think people like Mark should be intellectually honest and say, 'I believe in God, and it's not rational,"' he says. "If there really was a God-like being, I would be happy to accept that, because I want the truth. That's what led me down the path to atheism. If there is a God, I want evidence. I want good arguments, and they don't exist."