Many Theists make this argument at some point, but it was Carl Sagan who said it with the most clarity:
“An atheist is someone who is certain that God does not exist, someone who has compelling evidence against the existence of God. I know of no such compelling evidence. Because God can be relegated to remote times and places and to ultimate causes, we would have to know a great deal more about the universe than we do now to be sure that no such God exists. To be certain of the existence of God and to be certain of the nonexistence of God seem to me to be the confident extremes in a subject so riddled with doubt and uncertainty as to inspire very little confidence indeed.”
Carl Sagan was an amazing man and will always be my personal hero, but was certainly not omniscient. This single paragraph is a dazzling example of how a person could not be more right about the “relegation” of God, but could not be more wrong about certainty.
I personally find it puzzling that the average individual goes about his daily life perfectly content with practical certainty, and then suddenly insists on absolute certainty once he catches a whiff of philosophy or metaphysics. I am practically or reasonably certain that my car is still parked on the street where I left it. And if anyone asks where my car is, that is exactly what I will tell them. Many things are possible—it could have been towed, or stolen, or hit by a meteorite. Even if I were to look out the window and point to my car, that person might ask me if I can prove that I’m not in a dream, or in the Matrix? Or perhaps they would venture into solipsism, but I’d probably kick them out of my house at that point.
I am practically certain that Sylvia Browne is a fraud. She could possess psychic powers that just go on the fritz from time to time, but I’ve seen her give so much completely false information over the years that as far as I’m concerned, the case is closed.
I have the same level of certainty that Theism is false, for the same “relegation” reason that Carl Sagan was obviously aware of. Gods supposedly existed in trees until we cut them down, in mountains until we climbed them, in pharaohs and kings until they were deposed, in the sky until we invented hot air balloons. Even up until Sputnik launched, no small number of people were convinced that it would slam up against God’s crystalline spheres (or something like that) and shatter, because God of course wouldn’t allow it to punch through Heaven.
Now, in 2013, God is relegated to the “outside of space and time,” in what can only be called the Phantom Zone. If humanity ever does manage to explore this fictional realm sometime in the theoretical future, it would be a simple matter for William Lane Craig’s grandson (Billy the Third?) to invent an Uber-Phantom Zone, and put his cherished idea safely out of reach again.
I don’t bother with absolute certainty. It’s a red herring, it’s useless and most likely impossible. I’m interested in practical certainty, and have no interest in believing any idea that survives by virtue of being ill-defined rather than true.