Such a question is, of course, a bit sensitive and difficult for anyone to answer who hopes to avoid offense or argument. So I answered, simply, "I'm a Christian."
"Really?" replied the professor. "You believe that God has a son (which, of course, everybody knows is completely impossible), and that he sent his son to earth and arranged to have that son killed in order to buy himself off?" I replied that, while that wasn't exactly how I would have phrased it, I do in fact believe something along those lines. "Amazing!" exclaimed the professor. "How can any intelligent person possibly believe anything so obviously crazy?"