One of the big differences between religion and science as “ways of knowing” is that in science we can almost always specify what observations or experiments would prove our theories wrong. In contrast, the faithful do not (and cannot) specify what observations would disprove their beliefs—or the whole basis of their religion. There are two reasons for this distinction. First, through judicious theological manipulation the faithful carefully insulate those beliefs from disproof, often in a hypocritical way. When evidence is found against them, like the medieval age of the shroud of Turin or observations showing that prayer doesn’t work, the faithful simply say, “No, you can’t test God.” No matter that if the Shroud of Turin did date to around 30 A. D., or if prayer did cure people in double-blind tests, those same believers would trumpet to the skies the proof of their faith. Evidence for religious beliefs is counted; evidence against them is dismissed. Needless to say, science doesn’t—and couldn’t—work that way.

Second, because religious belief is irrational, the faithful often won’t let themselves even consider counterevidence. The evidence for evolution is by now overwhelming (I wrote a book about it, and didn’t even scratch the surface), but still around 60% of Americans think that humans were created by God directly instead of having evolved—and a lot of the latter believe that our evolution was guided by God. Faith has immunized these people against the plain facts. I’ve always thought that the existence of horrible tragedy and evil, particularly that inflicted on innocent people and that produced by natural forces like earthquakes and tsunamis, were prime evidence against the more loving and omnipotent species of god. But there’s a whole branch of theology—theodicy—designed to explain those things away. Let’s put it this way: if the Holocaust didn’t make people abandon their belief in God, then nothing ever will.

Religion is not a way of knowing because it doesn’t have a way of knowing that it is wrong. And without that, you don’t know if you’re right. That’s why science makes progress in understanding the world while religion is still mired in medieval theology.

Granted, some of the faithful—and many of you readers—have abandoned religious belief because it either seemed irrational or was contradicted by empirical evidence. Dan Barker has chronicled this journey in his poignant book Godless. But the abandonment of faith is often a gradual process, so gradual that believers don’t even realize it’s happening. How many of you, when you were believers, were brave enough to lay out the kind of evidence that would make you bail on God?

But we atheists, being scientifically inclined, can do the converse: we can lay out what observations would turn us into believers. Over at AlterNet, Greta Christina describes six things that, if they happened or were observed, would convince her that God exists. These including magic writing in the sky, correct prophecies in sacred texts, accurate information gained during near-death experiences, followers of one religion being much more successful (in ways that couldn’t be explained by economic and social factors) than followers of other faiths. Go read it: she qualifies and explains all of these things in detail.

Darwin himself, in a letter to the botanist Asa Gray, laid out his criteria for believing in God:

Your question what would convince me of Design is a poser. If I saw an angel come down to teach us good, and I was convinced from others seeing him that I was not mad, I should believe in design. If I could be convinced thoroughly that life and mind was in an unknown way a function of other imponderable force, I should be convinced. If man was made of brass or iron and no way connected with any other organism which had ever lived, I should perhaps be convinced. But this is childish writing.

Making the same point, I provided my own list in a critique of the claim that science and faith are compatible:

There are so many phenomena that would raise the specter of God or other supernatural forces: faith healers could restore lost vision, the cancers of only good people could go into remission, the dead could return to life, we could find meaningful DNA sequences that could have been placed in our genome only by an intelligent agent, angels could appear in the sky. The fact that no such things have ever been scientifically documented gives us added confidence that we are right to stick with natural explanations for nature. And it explains why so many scientists, who have learned to disregard God as an explanation, have also discarded him as a possibility.

It’s your turn. If you’re one of the faithful reading this, feel free to post those observations that would convince you that God doesn‘t exist.

And, if you’re one of the more numerous atheists, agnostics, or skeptics who comment here, supplement my list and Greta’s with observations that would make you accept God’s existence. If you comment, don’t be facetious. This is a challenge to those believers who say that their way of knowing is equivalent to that practiced by science and rational investigation.