I was raised to be a strong, convicted theist, and I'm still trying to shake off some nasty habits. Occasionally I see or participate in a conversation that trends towards something like SarahC's interrogation: “If there’s no God, what’s your system of morality?” “How did the Universe begin?” “How do you explain the existence of eyes?” “How do you find meaning in life?”

I've learned that an effective response is, "See what sort of interesting questions you ask as soon as you consider that maybe 'God did it' isn't a valid answer?" Then, you pick your favorite query and go into all the useful things humanity has learned as soon as they stopped treating scripture as a reason to stop thinking. Heliocentric astronomy is a favorite.

I cannot claim a lot of experience with other religions, but most Christians who ask questions of atheists are convinced they're "equipped" to handle the situation. Anyone with doubts would rather not talk about the subject of atheism at all. For the would-be inquisitors, then, introduce atheism as a useful thought experiment first. Discovering our ignorance isn't a "I don't know, but so what?" scenario, it's a "I don't know, isn't it great?!" sort of thing. While I'm still not completely convinced religion isn't without its uses, its greatest flaw is that it allows people to live their lives without ever learning what they don't know.