I tend to keep it simple when asked that by just asking in return "what if (insert religion besides the one they follow) is right?" It tends to end up in a quick illogical loop of backtracking, but it makes it easy to show how stupid the question was.
Hmm I wouldn't mind if I were wrong at all. It's interesting because it isn't that atheists WANT god to not exist - they just do not believe that he does. It isn't really our fault as much as it's "god's."
And I think spending eternity with Satan may possibly be better than with Yahweh.
What if Islam were the one true religion? What if Judaisim were the one true religion and Jesus was delusional or a scam artist or just mistaken? What if Hinduism is the one true religion? What if the Greek or Roman or Norse or Chinese or Japanese or Aztec or Egyptian or North American Native gods were the only true gods? (etc., etc. -- insert your favorite ancient pantheon) What if the Mormons are right? What if the Scientologists are right? What if the Summum religion invented by Clyde "Corky" Nowell in 1975 is the one true religion? Why don't you believe in the Heaven's Gate cult, Charles Manson's cult, Jim Jones's cult?
The same reason you don't believe in any of them, xtian, is the reason we don't believe in Jesus.
Think hard about that, xtian, and then let's discuss.